Hello,
I am LeifDaBeef. I am a Gold III Support/Jungle main and I play Thresh the most. That aside, this is a guide that will explain some advanced tactics you can do with Thresh but some beginner basics as well. I hope you enjoy my guide!

Pros-
Very good if played correctly
Good Utility
Good for catching low health targets attempting an escape.
Has an AMAZING escape for his teammates.

Cons-
Hard to play
Hook is easy for enemy to predict
Mana hungry early game
Without souls, extremely squishy.

I use-
Health Yellows
Attack Dmg Reds
Move speed Quints
MR Glyphs

These work the best for me, and the move speed is very nice and attack dmg reds don't require you to max E first to do decent damage.

I know, I know, my masteries aren't really "support" masteries. I get that. But I play so aggressively I need Thunder Lords Decree to be able to play my way.

Q-Your best friend, has a 1 second wind up, hooks them, pulls them back a bit, then lets go. You can press q again to go in.
W-Lantern. Other people click on it and they dash to you, good for going in on hooks.

E-Flay. Doing this backwards(for me) was the hardest thing on Thresh, because I was used to ones where it didn't really matter (like Blitzcrank's knockup). If used properly, it will benefit you greatly

R-Makes a box, and when people try to escape, they get slowed



Since Thresh is hard as hell to understand, heres a video. (Props to The Carry for the video.) Play Video

This is my simple guide on Thresh. Thank you very much for taking time to read it and have a great day!