Start with E:

your E is perfect for poke, it helps the jungler get his buff faster, he deals dmg to the adc/supp and can dispose the enemy.

Max R first:

There is nothing to tell about this, you need a low cooldown on this giant slow, it helps a lot in team fights and engages when the enemy has no flash

Next is Q:

your Q is the main ability of your kit, a huge CC that stun the enemy, pulls him and let you get near him so you can dispose him even better with your E and R.
The main reason why you're maximizing it is because of the cooldown that he gets. 6sec and if it hits only 3 secounds for a new one (with the 45%CDR)

Your W:

W is probably the most important skill that thresh has, it let's your allie get a good position, run from a fight safely and also shields.
Like Q this ability cooldown scales with the points that you give to it so you want an ability like this one every single (about) 5 sec.

WHY E LAST?:

your E has nothing special in comparison to Q or W, the only utility it has in mid/late game is the CC, it doesn't get cooldown reduction on level up and you're not going to deal any damage with the passive you alredy have what you need which is a good CC ability THE FLAIL B)

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FIRST PURCHASES

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It depends on the situation, if you're wining/loosing and you're wasting a lot of time, buy boots to roam mid and get some gold for your team (roam when the adc freezes or leaves lane).
The other two depends on the enemy team, if you need hp go for kindlegem, if you need armor and mana glacial shroud.
you will need these items further on.

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FULL BUILD

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Well before updating this guide it said "knight's vow is very situational", now I basically buy it every game, the boosts that it gives to your adc is a must, zeke's is a new add to our build, it's passive together with your ult make miracles.
Remnant of the Ascended, you only buy this later in the game, you don't need to waste the gold for such a tiny bit of hp!
The rest is situational, the heal and shields are good yes, but sometimes you'll need to be in the front line, in those momments you may want to buy something tankier

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THE REST

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Not much to say, if you get full ad enemy team go thornmail
if you need to go in front line or you get focus a lot (for example to deny ezreal Q on your adc) you buy gargoyle stoneplate, you'll become a wall
Zz'rot, if your team is being useless and you need to defend by yourself, or you're being useless and your team needs to push somehow, buy this, it's very situational, usually you're always usefull
Spirit Visage for long fights/trades and some nice ap on the enemy team
Adaptative Helmet, BYE TEEMO, BRAND AND OTHER AP CHAMPIONS THAT ARE ANNOYING AF, use it for massive ap on the enemy team
Abyssal Mask, OH BOY, DO YOU HAVE A LUX OR VEL'COZ ON YOUR TEAM???, DO THEY HAVE A RYZE OR SMTH?? this thing is op op! if you're the tank, then you need this for sure, unless the enemy/your team is full ad
Frozen Heart, looses to Randuin's Omen, cuz you'll only buy it for the enemy adc/yasuo, but it can be usefull if you need to sell your other cdr items!
Righteous Glory is literally the "GET OVER HERE" item, usually you get it when they have a sivir or a siege team comp, xerath and vel'coz will get their *** tightened once they see you running down at the speed of light lul

LANING PHASE:

Nothing big to tell you about, I mainly seek for Q's all the time, the adc will get pressured while farming and probably miss some, and ofc if you grab you're going to have a good fight, you need to be carefull thought! if your adc is weak early game and the enemy is strong you shouldn't engage before poking a bit or decreasing their hp somehow! If you start loosing somehow (enemy gank for example), just stay behind your adc and get ready to use your lantern, let him farm to get the gold and exp (be carefull not to get out of the exp range)

MID GAME:

Once you feel the need to leave bot, do it, swap lanes with top, go mid to help push the tower, be proactive! if you think you can get more gold on your lane then stay (for example tristana destroying every tower lul).

LATE GAME:

Mostly team fights and WARDING, wards are even more important now! You need to know what is the enemy team planning, are they going baron? are we ahead? yes? then let's contest!! are we ahead? no? let's do drake or push bot!!
In team fights you need to know what's your role there, are you part of the frontline or are you going to peel for your carry? pay attention to that!

I tried not to make it too long, but if you have any better ideas or you just want to spread hate say smth in the comment section!! thanks :D

I believe that you skip this part in every guide so I'm leaving it to the end.

My summoner name is WhiteGoliath and I just got demoted to g3 right now 18/6/2019 around 00:00!

Season 8 I grinded to gold5 by only playing thresh, it was pretty easy actually! I had some trouble with silver players but in the end I made it to get the orianna skin :D

Now in season 9 I wasn't in the mood to climb up to gold again, although I kept playing but with all the supp champions beside thresh... I was stuck s1! then I just said to myself "**** it" and perma lock thresh again, climbed to g4 and got hyped to climb even more!

Guess what? Thresh wasn't working... I was missing my Q's my adc wouldn't follow up and my brain gets damaged everytime I try to tell an advice to my team. So I went back to the all champions stuff, didn't get demoted but one day I locked Janna for the first time, my adc was a smurf and said I was good enough to reach plat3 or smth!

INSTALOCK JANNA BABY WOOOOOOOOOOOOOOOOOOO, and climbed to g2, but now janna is the one not working, I have an aggressive play style and I do a lot of bait plays, but the G2 players won't follow cuz it's a 60%/40% play... and now I got demoted to g3 cuz I played other champions besides janna and thresh ahah.

I'm bad but this guide was made by reading other guides together with my experience HOPE YOU ENJOYED THNX FOR READING :D

NEW: I was goofing around playing in others lanes using random champions etc.. and then "THE SEASON IS ENDING"---> OH SNAP! --> in the end I got g3 :P, but it was pretty easy so I believe next season I'm reaching at least plat4.