Short answer, yes you have to weld the hanger bearing plates onto the rear, then install the bearings and axle onto them.
Longer short answer: you have to figure out where exactly you want them at, figure the correct wheel base to track ratio. Then you have to line them up correctly and make sure everything's straight, square, and flush. Do not weld the hanger plates while the bearings are still attached, you can ruin the bearings from the heat.
You're going to need axle nuts, locking collars, and all the fun stuff as well, and are you planning on attaching your wheels to your axles?
Your going to need to do a lot of research to say the least. To start off with do some research into wheel base and track ratio for go karts. You're going to have to do some measuring and some math, and decide proper placement.
Sorry I'm on a time crunch and busy right now or I would write everything you need to know out, maybe someone else will chime in with more info.