dock

Hello! I have a little problem. If there are two crafts docked in space, when I undock the crafts, which craft I will be controlling? (Suppose that both have a crew capsule each) Is it randomly selected or what is given the priority?

I watch many tutorials on how to dock and I can get a pretty good approach and rendezvous, but whenever I start to actually dock, I just miss my target by 50 meters. Maybe answering these questions could help me. Thanks. 1) What direction should I burn in? 2) How powerful must my throttle be to...