That jungle tho
Not Updated For Current Season
So yes, this is a silly guide. But it works surprising well, bear with me.
So, to start off with, when to play Thresh jungle? The best time is when you don't have a crazy amount of crowd control on your team or if they have a weak damage early jungler. For example, if you're against Lee Sin, Shaco or Evelynn, or a few others, they're quite likely to attempt to invade you either before the Krugs or Gromp, or when you're taking the next buff.
Which type of Thresh build should I use?
Again, it depends what you're up against. If they have a bunch of people that will do a fair chunk of damage early or you haven't got much tank, go for the tanky build to help you survive. If not, go damage.
Where do I start?
Start wherever bot lane is really (or wherever you get the best leash). Preferably you want a support with a Relic Shield to ask if they can tank a couple hits for you since they can then heal with Relic Shield. Both blue and red buffs are very useful for you. Since you don't have a lot of early damage, red buff gives you more damage on your auto attacks (they're very important). Blue buff gives you cooldowns to mitigate damage better, as well as keeping your mana topped up.
Builds? What is this?
The most important thing in both the damage and the tanky build is COOLDOWNS. This is because it means you can Flay and Hook more often to stop people moving anywhere (good for ganks and teamfights). You'll need to be careful about how things go, but Talisman is great for mana restoration early on. Tracker's Knife is what I go for since the bonus AA damage and your Flay bonus damage stack nicely. As said in the builds, boots are dependent on how much you gank and if you need the CDR.
Try to steal them if you can by hooking their jungle if possible. It means they can't Smite, so you can. Easy steals.
As for Runes, if you're playing tanky pick up standard stuff in Health, Armour and MR to help you out. Don't know how to make a second rune page in the guide so go with that :P
Most importantly, have fun! This build IS silly, after all. Though it has a decent win rate...