How do I play Thresh ADC?

You don't. Your autoattack steroid involves not autoattacking for a while because it's actually a poke tool and not a steroid, your Q stops your autoattack, and you have no AD scaling. And every time ADC Thresh sort of almost works, Riot nerfs it.

You don't. If you really must do it try picking against AD heavy teams such as Talon/Zed mid. Death's Dance's Omnivamp works with your on hit damage. For crit items you can go Cannon for fully charged E poke, or Hurrican for AoE dps.

Get a tank support Max e and wait for it to go red then smack the enemies with dark harvest Get firecannon ie duskblade and then whatever you want. Lose the game because everyone else is feeding and you fall 2 shot but then realise it was worth it because you had a lot of fun.

Use your right click

I've seen players do well with because of his e passive.

Dark Harvest, Cheapshot/Taste Of Blood, Eyeball Collection, Ravenous Hunter; Legend Alacrity/Bloodline, Coup De Grace

or

Press The Attack/Fleet Footwork, Overheal, Legend Alacrity/Bloodline, Coup De Grace; Cheapshot/Taste Of Blood, Ravenous Hunter

Get a duo. Offmeta makes a lot of players tilt and not play their best. I've played with and against Thresh ADC and it's pretty bonkers once they get . So much stick and tank.