As far as teammates following up, don't count on it unless you guys are ahead and you've noticed they like to chase or go all in. So otherwise, I would just peel for your carry if you find yourself alone. Nothing you can really do there besides ping enemies and ask them to follow up if you go in; they can always choose not to listen however which would put you back to the previous suggestion. To avoid going in by yourself, you could always be a follow-up-initiator, waiting for someone else (hopefully a tank and not a squishy) to engage, and then follow up with your superior cc and peel.
Lack of damage as thresh is supposed to be there. He's a support with a lot of cc and utility power instead of damages. I wouldn't worry about not being able to finish enemies off as much; but if you really want to, a frozen gauntlet might not be a bad idea or just make sure you get your souls for more ap = damage. I don't know which videos you watch, but I'll take a shot in the dark that you might be doing vs what they aren't. They might not be auto attacking in the midst of the fight, maybe once at the start and then loading up the flay passive to be able to finish people off when they think they are safe. (Might be wrong here though.)
As far as looking for hooks, when to flay, and positioning, that is a bit trickier.
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Always looking for hooks is great. Hooks + good follow up = "An Enemy Has Been Slain". But if your team isn't in a good position to follow up, or like mentioned above, don't like to follow up, then save it to peel.
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Flay is often used in a limited way. Q, lantern, Q, Flay. Pretty common right? There are SO many more uses for it than that. You can use it to** position your enemies to better suit your team's needs**. AoE abilities or ults for example, a flay can secure an extra enemy or two to be affected which can tip the scales in your favor just enough to win the fight. It can also be used to **deny gap closers of enemies ** such as an Aatrox jump or Leona Zenith Blade. Deny channeled abilities is another excellent use of it if you can manage it.
**Be careful though,** flay can just as easily displace enemies out of allies' abilities/range in a negative way, hurting your team. **Awareness is key**
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Positioning with Thresh is everything. Again, Awareness is Key. You need to determine what your team needs more: does the enemy team have a devastating channel that needs to be interrupted? Get where you can flay/hook them. Do they have assassins that can jump your adc? Stay relatively close to them so you can flay them off/hook/lantern your carry away to safety. Are you the initiator? Be in the front and watch for out of place enemies. Etc...
Several of the Big Plays Spotlights has a thresh in them (go figure lol) so I'll link you to one of them that features Thresh specifically and then feel free to look at some of the others to see what they are doing with Thresh and think about why they are doing that as opposed to other things. Hope this helped somehow, good luck!
http://boards.na.leagueoflegends.com/en/c/riot-official/wqjL70WW-big-plays-big-flay