For those of you who came here looking for a detailed thresh guide, i am sorry to dissappoint, this isn't the guide you were looking for. This is an easy exploitable strategy that will get players out of low Elo as a thresh main. If you want to learn thresh but are low elo, i would suggest taking another guide, learn the basics and then come back here. Therefore through out this guide i will assume you know how to play a little thresh.

As detectable by the build, and descriptions, locket is the core of this build, but why?

Its a huge shield that stacks with your HP,
Thresh gets passivly armor so by building locket you can easily get a lot of tankiness, as your build focuses on HP, your Armor comes from your Passive and locket provides MR.

Never fight without your locket, and make sure to use it right before an enemy combo, or a teammate dieing. Keep an eye on teammates like Kayle or Tryndamere that you dont put locket over their Ultimate.

Treat this item like a Hook, in short it replaces your hook as a engage tool.
Activate rightous to run in and flay an enemy, therefore leaving your hook to or lock him down, or for the remainder of the fight.