I know ! I know what you're thinking: OMG Thresh AD Carry what the hell is this guy doing ?!?
And I agree guys, even if I use to play ranked games, this guide has been made only for fun, because I found Thresh is incredibly good as AD Carry, so don't judge me. It's just for fun, but it's viable !!!

PROS:
-> Incredibly funny to play
-> Q passive added with sheen makes nice damages ^^
-> CC every where ! = assassin's style
-> passive no com...
-> pretty good begin

CONS:
-> Not serious adc ^^
-> Can be quickly kicked out if you're outfarmed

The item order depend on your ability to farm !
Thresh has a very good begin as adc so you can keep your lane for a time, and you're items order will depend on it.
I personally don't pick any Doran's Blade but it's also a good option after boots+popo or even at the beginning.
After getting your BloodThirster the item order is not important, it depend on how do you control or not your lane, for more safety, begin with Phage, if you need more mana, go Manamune...

I chose a very offensive 26/4/0 but It's also viable and maybe more intelligent to take 21/9/0 for more safety