This is a thresh guide updated as of patch 7.4. With this guide, I will offer tips on match-ups, what to do in game, what to buy, the purpose of good skill order and some bonus tricks to maximize Thresh's potential!

2x Gold per 10 quints. 1x Armor Quint
You need all the gold you can get to stay ahead of your opponent. Your goal is to outclass the other support early, and try to get your team an advantage. Money doesn't buy happiness, but it does buy early sightstones!
8x Flat MR Blues. 1x mana regen per level
I like flat bonuses because the pressure I apply in my games comes from early-mid game, which is where flat runes are favored. If you know ahead of time that you are playing for late game, I suggest being more passive in game and taking scaling runes.
6x Flat health yellows. 3x flat armor
Health or armor? well, I like health because you are going to be squeezing value out of potions in lane to press an advantage and you are less inclined to burn pots if you are missing "extra" health. This saves some money for you to get some early items.
6x Flat AD Reds. 3x Flat Armor
Laning is my favorite part of every game I play. And I like to take risks and get kills for my team so we have a better time in game. Later I'll talk about trading, but for now, damage is going to be needed to achieve lane dominance.

The masteries I have listed are pretty standard, The only one that I use compared to many other builds is my 12th point in the resolve tree. I choose perseverance because of my play style, which involves many trades early. This point is used for personal preference. If you like to make flashy madlife hook attempts, I suggest taking the summoner spell cd reduction, but the build has lucidity boots, which offer almost the same benefit. The other option, the protections, leads me back to my armor vs. health argument, where I like having health over armor and mr.

Relic Shield will be the start for my thresh build. I'm not a fan of coin, simple as that! The goal of your first back will be to purchase sightstone, basic boots and pots and at least 1 pink ward. Hopefully, you have more gold to toss around though!
After Sightstone, you need to figure out which of 2 items you need to go for next. Either upgrading relic into face of the mountain, or building locket. I think a shield item is 1000% necessary to your team during early-mid game. If the enemy team has a lot of aoe dmg or high magic damage, get locket. Examples would be a team with a ziggs, wu, malphite, zyra, or singed. If the enemy has high single target damage or you have a good champion that has a strong gap closer, then get face of the mountain.
Everything after these few items is situational.
Frozen Heart: Frozen heart should be bought when a good portion of the enemy damage comes from aa's in long skirmishes.
Redemption: Top tier support item. Good stats and good active.
Banshee's Veil: should be bought against high poke comps and high magic damage teams.
Randuin's Oman: If you are the only person on your team that looks at the enemy builds and sees armor. And your team fails to get pen, because they are silly.
ZZ'Rot: I like to buy zz when behind. You will need to help apply pressure so you don't get steamrolled and lose too many objectives and map control. Also, if it is very late and you have "more than full build" gold. You can sell boots, buy a zz, put it in a lane, and go base and rebuy boots.
Zeke's: If your carry is getting ample peel from everyone and is fed, this is the perfect item to aid the stomp. Don't buy it if your adc is weak sauce though.
Banner of command: This is another item to buy when behind, but get this instead of zz, if the enemy team is reliant upon spells to deal damage. And remember, the cannon minion in a super wave cannot be banner'ed!

Q first. We like to invade. Most of the time, you can burn at least 1 summoner spell or secure a kill.
E second. We will be bullying our opponent in lane when we can, and having the empowered auto's earlier is better.
W 3rd. This rounds out your kit.
E points for levels 4 and 5. Early on, your damage will come from your aa, flay, aa combo. Maximizing this damage is important. I also like to do this because Q cd, even when hit, is ~15 seconds,and short skirmishes benefit more from E leveling.
6: R. duh.
7-10: Here, we max our Q. It is mid game and the utility is more important than small messy skirmishes!
11: R
12-15: W. Again, mostly for utility purposes. Your E is no longer a damage source out of early game and it's purpose is to cancel/impair movement. charges, jumps, dashes, lunges, etc.
16: R
17-18: E.

Flash is a must.
Exhaust or Ignite?
Ignite offers more early game kill pressure, but is useless in this meta mid-late game.
Exhaust is move versatile all around and with a lot of adc's getting nuked by assassins, doing what you can to avoid that from happening is good!

Just an intro, practice mode helps learn ranges and things/techniques I'll be talking about.
1st Tip: Always landing your Q. A lot of people don't like burning summs like flash because of it's value, so hook towards the sidestep. This normally means opposite of your position. (Hooking from red side lane to blue) Hook lower than where the champ is walking. Vice versa for blue to red.
2nd Tip: Hitting a hook, dropping a lantern for a teammate, then taking the hook. this takes time to get the key order down, but it's great for making picks.
3rd: Caster minion flay. The back line of minions is the caster minions, meaning, not melee. If you stand in front of them, flay them towards you, they land behind you and you can send an unsuspecting hook towards the enemy.
4th: Hook predictions: Nothing but time can help improvement of this.
5th: Communicate with your adc to rush lvl 2. Most of the time, if your team drafted well, you have an even or better lvl 2. I try to get a lvl 2 all-in every game. It helps with controlling the game from that point on.
6th: Learn ranges of Q. And try for max or close to max range hooks. The distance is a lot longer than people assume, and if you hook from "far away" they might not expect the incoming range. (Practice mode will be good for this)
7th: Minion Taxi. Sometimes it's good to miss death sentence on purpose. This gives the enemy a sense of reassurance because "thresh missed....clearly that play is over". just drop a lantern, and take the "missed" hook. This will often catch the enemy off guard.
8th: Patience is sooooooooooo important. Make the most out of your awesome kit. Save E to maximize cc and stop movement, and just because an ability can be used in a fight, doesn't mean it should be used asap. Save a hook to try and peel, or save lantern to help a teammate get out. Often, you can use your ult to stop a chase, and protect your team. Just be patient!
9th: Improve game knowledge. What I mean by this, is think, "If I do this, what are they going to do?" This is how I beat many morgana's in lane. If you walk up from brush, flay the adc and aa them. Odds are, the black shield is coming out soon to stop the lockdown, knowing this ahead of time, let's you and your carry know to switch focus and try to kill morg instead. Or "if I flash, where would the enemy move to to maximize safety?" and try to improve hook predictions off of that.