First thing what I do is checking my patrol grid on the Kriegsmarine map.Then Ill check the navel route and Spinnennetz maps for merchant / convoy routes and plan my route near those sea lanes and outside the air cover range.
I leave all shipping alone in the North Sea, those ships are for the II's and VII's. Unless I'm participating in some operation like Weserubung.
I leave lone merchants alone as long as possible. Untill the enemy gets too strong making the loners an easier choice at that point.
Favourite spots to hunt for convoys are the Dodger's Bank and SSE of Rykjavik. I'm a North Atlantic raider
I tend to shadow convoys more and more before going in for an attack. Get to know where the big fries are before launching an attack. Also very usefull for Spinnennetz
I quietly go in submerged and try to get as quiet out of it. Ill surface the boat and do a parralell run alongside the convoy to set up for other attacks. (Note that in the summer theres hardly darkness in the North Atlantic)
One tactic I always wanted to do but never have done. On a light escorted convoy torp the flank escort first with one shot. This will make nice room for a (submerged) raid on the convoy later on.
On a failed convoy attack but when a ship looses speed Ill wait a bit and finish it off with the 88 if the weather is good.
And when all get hectic and a DD is chasing me. Up periscope and let the DD chase me on a 180 bearing. A fast magnetic torp is fatal for them at 300/500mtrs.