Most of the time, you are going to play versus melee champions. As a ranged champion, you have a huge advantage. Harass them when they CS at early levels and be careful of minion aggro. When they try to trade, just Flay them back and repeat. Keep in mind that you are basically a champion made for cc, so you have no damage from your skills.

The damage in this built, comes from:

1. Flay passive/ranged harass with autos
2. Bami's Cinder/ Sunfire cape
3. Ulti damage (souls passive + base damage op)
4. Jungler

That means that you are weak early without a bami's or a trinity. You may need to farm safely for them and until you get a phage and bami's, you will feel weak. The good thing is that if your jungler isn't braindead, he will understand that you are actually not trolling, playing serious and that you are the best lane to gank to give him kills with the massive amounts of cc you provide. Also, no enemy jungler in his sane mind will dive you if you farm under turret for your early items. If anyone dives you, just unleash your suppressing combo and chances are you'll get at least 1 kill.

After you get phage and bami's, you will see that you start to feel strong in the trades and you are. Finish boots for resistance (if you need it) and go for trinity. The trinity powerspike is when the enemy top laner will start typing in all chat "WTF". Trinity makes you very strong in 1v1s and keep in mind that you are a ranged tank champion.

For the all-in, you want to get as many trinity procs as possible and make them hit 2 ulti walls with your flay for a guaranteed result. You have a cage that does decent amounts of slow and damage, a flay to displace and slow them plus a hook if they try to run. if you play VS a melee champion, keep them always in auto attack distance. They will never auto you with your CC and they cant run away with your hook. Dodge their skills and outplay them with flay. Mid game is your damage powerspike, use it to your full advantage to take a lead for your team. I always build a Zz'rot if i have an advantage in my lane to take as many objectives as i can with it before i fall off damage-wise.

Since you go full tank afterwards, you won't have much late game damage just with trinity. You will have at least 100+ armor +ap from souls and 30-40% CDR on a CC machine though. Thresh's Q and E are broken with that much CDR, its almost like playing URF. Your box will do much damage and lock people with the slow, your lantern will provide with a pretty good shield, you can hook and bring someone in, you can do whatever you cant do with a much weaker support thresh. In the very worst you can perma stun 1 or more people in a teamfight, best case scenario you hook a carry and permastun it until your team kills it and you do decent damage to it in the meanwhile.

TLDR You are a ranged full tank with a passive built for tanks. You can permalock at least 1 person and do good damage to it in a teamfight while you're tanking, or make a pick if your team is loosing and turn the game around. Plus mid-game you're a beast!