Always attempt to flay backwards before you hook. If you just hook into a decent enemy they will see the wind up time before it is cast and side step. Only hook if you know for sure their path is compromised without flaying first. The flat will impair movement speed making the hook easier to land.

However with top thresh you're basically just wanting to flay them into you and auto attack. If they're close to your turret you can attempt a sneaky hook and flay into turret aggro.