Hi, It's my very first guide for a LoL champion. I hope you find this useful.

First of all, here are some words about Thresh the Chain Warden.

Thrash is a highly tankie support. Thresh does not gain armor per level. Instead, Thresh collects the souls of dead enemies by walking near. Souls permanently grant armor and ability power. Champions and large minions always drop a harvest-able soul. Small minions only sometimes drop a soul.
A soul will only drop if the enemy unit dies within 1900 range of Thresh. Souls are visible to allies, and only become visible to enemies if the enemy team has vision of Thresh.

From: official wikia:

The first soul will grant 1 bonus ability power and armor. Subsequent souls will grant 0.005 less, down to a minimum of 0.5 per stack. Souls cap at 999999. [1]

The 0.5 minimum is reached at 101 stacks, granting 75.75 bonus stats.

The maximum amount of ability power and armor gained is 500,025.

This amount of armor, including his base of 18, grants 99.98% damage reduction.

Even with the 35% armor penetration from Last Whisper, Thresh still benefits from 99.96% damage reduction.

Thresh surpasses 99% damage reduction at ~20,000 souls, rendering souls above this surplus with regards to armor.

It takes 49-94 souls to catch up to the armor levels of other level 18 champions (61 to 90 armor, including his base armor).

At 95, Thresh has the highest innate armor not including other champion's abilities.
It takes 182 souls to match a level 18 Taric with rank-5 Shatter - the highest armor available without items, runes and masteries (134 armor).


suming up the above: Thrash can suck if you don't position well enough.

Pros:

[*] Gains souls without having to kill a single minion.
[*] Very high armor and magic resist.
[*] Very good support at near every situation.

Cons:

[*] Does not gain armor and magic resist by levelup.
[*] Lacks healing skills (it's a con as a support i think...)
[*]

Communication is very important. You need to chat, ping, or skype to keep up the communication with the teammates. You have to signal each other when and where to position yourself and others. You and your team has to move and work together more than with any other character. They need to understand your tactic, and you have to understand theirs.

You only need to farm souls. If there is a big minion farm on a lane, go there with a good crowd-exterminator, like Kha 'zix or Talon, or anyone with high damage AOE attacks. He or she takes the kills in a blink of an eye, and you just collect the souls remaining. But that is important, that your partner shouldn't rush forward, and kill everything. This way both of you takes advantage of the same farm.