Thresh and Twitch is a great kill lane combo by level three. Twitch just has to be in ambush mode as thresh hooks. Then Thresh leave a lantrn and runs, so that Twitch can flee if needed. And of course hit them with a flay before you run. But I understand this can be out played.

This is incorrect. The execute does not work for him but he can still use Spoils of War. His passive from flay allows you to last hit more easily than other ranged supports. This is an added reason to max flay first. This item is also the most gold efficient of the gold generating items being over 107% efficient even without using spoils of war or its active.

Face of the Mountain is outdated (the passive works with Melee Attacks)

You just gain the extra gold when last hitting because it doesn't include the execute damage.

Runes are Season 3: Greater Quintessence of Armor + Greater Seal of Health or Greater Quintessence of Health + Greater Seal of Armor are standard right now. After finishing Sightstone you can already switch your Trinket to Sweeping Lens and buying Face of the Mountain before Aegis of the Legion is viable too and maybe more optimal. Your fist backing items are a bit weird because the two Regeneration items aren't needed. Masteries shouldn't contain Enchanted Armor or Perseverance , mathematically worthless.
In terms of skill sequence you can max out Death Sentence straight after putting three points in Flay ...

For what it's worth, Thresh's attacks do trigger the passive. I'm not sure if Thresh's attacks count as melee attacks (after all, he is swinging a weapon rather firing a projectile) or whether there's a special case for Thresh, but it does work.

Nice guide, I like this build. Voted!

Very polished guide!

Very polished guide!

Very polished guide!

This build has really helped me as I learn to play Thresh. I usually go for Soraka as a support, because of her great sustainability and harass strength, but Thresh is really fun to play too and you've helped me better learn him. Great job.