Hello there! Thresh is my most played champion having an ~70% win rate with him in ranked. This is how I play Thresh and while there isn't the best way to build and use him, this I found works most effective on him.

Thresh was the first champion I learnt absolutely thoroughly and the first support to put my hands on. At first I believed that Thresh doesn't deserve to be a support, not being able to do damage, but if you hook, pull and flay your enemy, you're condemning him. His kit is very powerful in that way. And his The Box is one of the best slows in game, slowing his enemies for 99%. In game, unfortunately, champions can't be slowed under 115~ MS.

Pros:

+He's considered one of the top 5 supports in season 4.

+His kit is very good for carrying as a support.

+Doesn't fall off late game. He's the only support that scales infinitely.

+Has CC on almost all of his abilities.

+His lantern grants a long range Flash, perfect for making plays or saving a teammate.

+Fits in any team comp.

Cons:

-His base movement speed is very low.

-Doesn't gain armor/level.

-Might be risky picking up souls in lane, making him susceptible to hard harass from enemies.

-Becomes useless if he doesn't hit his abilities.

-Needs time to perfect and adapt to his playstyle demand.

Pretty straight forward, the runes stack nicely with your passive and gameplay. (You can adjust and replace. I play Thresh very aggresively in lane so AD runes fit my gameplay.)
My choice:Alternative
Or you can choose Gold/10 Quints


Personally, I take more AD because items and masteries grant me more than enough tankyness in lane and teamfights. And it scales disgustingly well with his E passive.

Please note Thresh doesn't gain armor/level due to his passive, Damnation. You need to take the souls the minions drop and if you have a hunch your laning phase will be shameful and soul-less, you're better off picking up Armor Quints.

Defensive tree in the masteries helps Thresh a lot more both with his laning phase and his teamfighting.

I take the cooldown mastery Sorcery in the Offensive tree for obvious reasons - you want to spam your The Box and your Death Sentence a lot more.
In the utility tree I take the mana regen mastery Meditation . As a fairly experienced Thresh player, I know how to save my mana in lane, yet it helps a lot after a missed ability/hook to recover the wasted mana much, much easier.

I take a point in Phasewalker as well. You can always take the Scout mastery over it, though I find it more useful to get the enhanced recall mastery.

Worth mentioning is that Thresh benefits decently well from the Fleet of Foot mastery. His base movement speed isn't amazing, so these 3 points can be as placed in the movement speed bonus rather than mana regeneration if you're confident enough you're not going to spam your abilities.

Down to the Defensive tree, most masteries I have listed are very useful for Thresh. As the game progresses, you will want to turn into utility tank initiator/disengager. I find useful in the laning phase that Block and Recovery are really good because between levels 1 to 5, the majority of the trading in lane will be from autoattacks (this will also spare you from starting with Doran shield, because the Doran start lacks any form of vision, unless you count the trinket, and an early gank can lose you the lane).
Once you take Block , you will want to take Unyielding as a must. Self explanatory, as a ranged support you reduce all incoming damage by 1.

Veteran's Scars is such a good mastery. The 36 hp at lvl one is really good, even if your team will invade, even if not. You can help your jungler tank a few Lizard/Golem hits, you can receive an extra hit from your enemy laners and still have enough health without having to back.

If you take Veteran's Scars , you take Juggernaut . Once you're done with building the MR/Armor, you'll start building health. Health goes a long way, while MR/Armor not so when talking about late game, and this mastery only helps you with that. Also comes in handy when you start building the Sightstone with the Ruby Crystal. Grants 3% of those stats for 1 mastery point. Very good imo.

Most botlane combos are made of the ADC which deals physical damage and the Support which is generally dealing magic damage. I put 3 points in Hardiness and 1 in Resistance . Helps with survivability in lane. As Thresh, with these masteries even if you fall one or two levels behind, you won't die instantly.

From this point, the next masteries will help a lot more with ganks, skirmishes and teamfights. The Swiftness mastery works really well against slows. Also this for only one mastery point? I'm in.

Tenacious is again another really good mastery, also very helpful with the famous 4 man gank bot, since it grants +4 armor and +2 magic resist for each nearby enemy. It's like Stone Skin but in the masteries. I find this really valuable on Thresh, giving him a lot more survivability in teamfights, where you want to apply as much CC as possible before dying. You take 4 points here.

The last mastery is as well really, really good: Legendary Guardian grants what we call Tenacity. This mastery could make the difference between a successful teamfight and a bad one. Grants 15% crowd control reduction. Pair it with Mercury Threads and you have all crowd control halved (for those who aren't aware of this, Tenacity as an in game stat doesn't stack; only masteries + 1 tenacity item). Fiddle Terrify will only last 1.25 seconds at rank 5 and Annie Pyromania only 0.90 seconds. Sounds worth to me.

When playing Thresh I always prefer getting Exhaust and Flash. Rarely I'll get Ignite.

You take Exhaust most times. It reduces Attack speed, Movement speed and damage output. One of the best summoner spells for supports in lane and while teamfighting.

Flash is a summoner spell that is key to my gameplay and this tutorial. You engage with it, you disengage, and very good for escapes. Best summoner spell right now in game, would probably never take Ghost over it. You might consider taking the Distortion enchant for reducing this spell's cooldown.

Ignite is one of the situational summoner spell you take against champions with health regeneration abilities. Personally I take this spell against champions such as Dr. Mundo, Nasus, Fiddlesticks. Sometimes I take it even against Annie. Her burst is insane, and while she doesn't have any kind of hp regen ability, Ignite helps shutting her down early.

Honorable mentions:

Teleport is a decent choice of summoner spell on supports. Not recommended unless you have at least one exhaust and one ignite in your team, but could be good for setting up ganks for toplane or helping a lane out.
I felt I had to put Teleport here because I've seen a lot of players actually take this spell, but I don't think it's as rewarding as the other afore-mentioned options.

Heal - not bad but not very useful in the current meta
Revive - wow no
Cleanse - you can get 50% tenacity and that's more than enough
Barrier - don't need it
Ghost - Flash is always a better option unless you're Singed, Dr. Mundo or Tryndamere
Clarity - you don't have mana problems big enough for taking this
Clairvoyance - you have trinkets, it's not season 2 anymore
Smite - this was a support guide last time I checked

I think it's needed to bring an explanation on his skill order.

I'll be frank, it's very very situational. If you're ahead in lane or it's an even lane, max out Flay first. If not, max your Dark Passage. Your E slows and knocks back enemies, and it's your only ability that deals damage (at max level it scales as 200% AD) and deals magic damage based on how many souls you collected.

Example: If you collected 200 souls you'll deal about 400 damage per hit when the icon in your status bar turns red. If you level this first, your harass will outscale any other support's except mage supports, such as Annie, Syndra, Orianna (the late aren't very often seen as support picks but they are picked in SoloQ at higher ELO).

If you're behind, or your client crashed and you had to reconnect and your enemy laners are lvl 3, and you're level 1, consider maxing your W Dark Passage first at lvl 9. Your shield amount increases with every point (60 / 100 / 140 / 180 / 220). If you decide to go ham when you're behind, the outcome might not be in your favor.

Your Death Sentence is your Q ability, and the only thing you gain from leveling this is some cooldown reduction on the skill and magic damage increase when using it. I prefer maxing this last, since the CC doesn't change after leveling. The stun remains 1.5 at all ranks.

Your starting items will have to differ whether your team is invading or not.
For invades:
3x Stealth Ward, 1x Vision Ward, 3x Health Potion, 1x Mana Potion, 1x Sweeping Lens or Stealth Ward
When not invading:
1x Ancient Coin, 1x Stealth Ward, 1x Health Potion, 1x Stealth Ward

At your first back you'll want to get a Ruby Crystal or Sightstone if you have enough gold, Health Potions and Boots or another Vision Ward. Vision control is important for ganks and your lane's safety.

Keep in mind, you want to update your Ancient Coin as soon as possible! Rank 1 grants +2 gold per minion, Rank 2 grants +4. I'll come back on this matter later in this chapter.

Due to Thresh's flexibility and Damnation, he doesn't need many items to be tanky. You can continue from here with situational items.
Boots

- useful against heavy CC and/or hard engage teamcomps. Also not bad against fed AP carries.
- fed AD champion? No problem.
- need more hooks? Take this.
for making plays, escaping, roaming to mid. Sell these and get one of the ones above after the laning phase is over.


Situational choice:

You can stick to your talisman of ascension until the end, or you can get face of the mountain.



Worth mentioning on the items list:

If you find yourself being focused (it happens a lot at lower elos, to get focused as Thresh, happened to myself) these items might come in handy. Guardian Angel is pretty self explanatory, gives MR and Armor, not a bad item, but I'd advice NOT taking it unless you go out like a light.
On the other side we have Banshee's Veil. Not a bad item and most probably you'd like taking this against very fed AP carries such as Kassadin, Fizz, Syndra, Ziggs, Orianna. 9 times out of 10 you take this over Guardian Angel when you're playing Thresh.

Trinkets

In the laning phase I always prefer Stealth Ward when I start with a gold item - Ancient Coin, though after the 15-20 minute mark you will want to sell it and get a Greater Lens, then update it as soon as possible to Oracle's Lens.

Oracle's Lens is a very powerful item right now. The active grants 10 seconds of true vision when upgraded. When your team decides to Nash, this item can sweep completely the Baron Nashor area. If any place remains unsweeped you can place a Vision Ward to scout for other sneaky Stealth Wards.

Your level 2 as Thresh is very poweful. Use that to your advantage.
Your level 2 power comes from your Flay and your Death Sentence. If you miss your combo, let it go. Don't continue with Flay because you'll probably lose that trade.

When harassing you might want to stay in the lane brushes and go out to harass when you Flay passive icon is red. When the passive is red, it simply means you do extra damage.
80 / 110 / 140 / 170 / 200% AD + number of souls

While still in laning phase you must remember, you're a support with a very strong kit. As Thresh you want to be the one to harass when your passive is up, you want to protect your carry even if he sucks. You die instead of him. Shield him and be his meatshield. This is the reason you took 22 points in the Defensive mastery tree. 8/10 times you'll even survive, while your ADC could have died.

Reaching level 6 before the enemy support means you win the lane most times. Ping your carry or let them know you're gonna engage and proceed.
Death Sentence >> Flay >> The Box and then wait for your carry to take the double kill.

Using your The Box as soon as you get it it's a very interesting strategy, because no one expects someone to use their ultimate right away. The element of surprise makes the targets run away, fighting back being no longer considered an option.

One more piece of advice while the laning phase, always keep Dragon and the tribush warded (on the Blue side). These are always ways the enemy jungler will use to reach your lane.
If you're against Zac, you might want to consider over walls or the Purple side tribush (when you're on the Blue team) because I noticed Zac players prefer that route.

As Thresh you'll be playing two roles: initiator and disengager. Your kit can serve well in both situations. When your team is ahead in kills and gold, or if the enemy adc/apc isn't in position, you'll want to initiate.
My favorite initiation is Flash + The Box. Your ultimate skill deals a fair amount of damage to squishies late game, your team can fly in and kill key targets after you already chunked 10-15% of their max health.
Your Death Sentence plays a big role in cleaning up, supposing you're still alive. If you succesfully killed their ADc and Jungler, but the AP carry managed to hide in a brush you can throw your hook even over walls. Death Sentence + Flay combo will always give your team enough time to reach you and pin the said carry down.

Another way to initiate can be simply walking into the enemy team and setting your The Box up. In this case you'll have more than enough health to survive. After this, your enemies are most likely to run. Flay them back into your team. Then feel free to Death Sentence any key carry that managed to escape.

For out of position carries you can use a combo such as Flash + Death Sentence. Unless you're in Diamond, this should work most times. If it misses, go back, it's no use going back in with Flay and 2 cooldowns.

When a fight goes bad, you become the disengager. You might ask, but how?
Your The Box skill has a 99% slow as previously stated, and your Flay goes 2 ways. Flay can push enemies away or it can bring them closer. When you disengage you want to Flay them away, putting enough space between them and your carries. While it's preferable for you to survive the disengage and run back to base, never let your teammates die so you can escape. You can slow them down by using an alternate route. Your ADC flashed over Baron wall? Run the other way. Most times the enemy team will chase you down instead of the others, so your carries can heal/shop/defend until you respawn.

As Thresh you might find useful to know you can hook on camps you don't have vision on. If you're getting ganked, and the enemy jungler keeps chasing you, you can hook on the wight and reactivate Death Sentence to pull yourself over the wall. This works with any kind of jungle camp and even Baron Nashor and Dragon as long as they're spawned.
Careful when you're low health, because the jungle monsters can kill you even if you escaped from an enemy player.

Smartcasting as Thresh is very useful, though it might take some time getting used to. The hardest skill to smartcast is by far Flay and I don't recommend doing so until you know exactly how its mechanics work.

A common way to help your jungler gank is by using your Dark Passage and Flash. When your jungler clicks your lantern and he's flying towards you, you can Flash and it will teleport you AND your jungler to the target location. Then Death Sentence the enemy ADc. This technique doesn't require much map control since you can help your jungler come from a screen away, from your own lane.

Technically you outscale any other support, yet your hardest matchups will forever be Leona and Annie. You can counter Leona's Zenith Blade with your Flay if you time it correctly. I've been told it's not possible, but I have succesfully did several times.

This is my first guide ever to make, I hope I gave enough details and that it isn't exactly the same as other guides.

Keep in mind this is my way to play Thresh. It's a very aggresive Thresh, and I don't recommend doing any of the stunts described in this guide until you're at least comfortable with playing this notorious champion.

Comments and constructive criticism is very welcome. Even though I tried to stay as true to facts as possible, there can be errors here and there. Feel free to point them out and I'll edit the passage.

Thank you for reading!