Heya. Measure the handlebar-clamp part of the stem to get the bar diameter. It's usually in mm.
As for pre-drilled/sub-tape, it's your preference or whatever you can find. I just wrapped my brake cable underneath. First I taped the cable to the bar really tightly with duct tape so it wouldn't move around while I was wrapping. Then I just wrapped over it. Common sense though, make sure to leave plenty of slack in your brake cables so you can turn your handlebars, haha. I don't know of any bar wrapping tutorials, but I always wrap inside-out, that way I get really clean starting lines that don't need electrical tape. Lift up the brake hood and fiddle the tape around that, then just flip the hood down. Looks good man. It kinda takes practice, maybe five wraps to get awesome at it, but it's really not rocket science. It's kinda hard to do a TERRIBLE wrap job unless you're drunk and in a mad rush.
Sorry, can't really help with bullhorns, I ride drops. I'm pretty sure it's all very doable; a handlebar is a handlebar, right?
Hopefully I helped a bit!