It will help if you can visit a new house being built or visualize how the studs are laid out in your house. The studs go up and down and create hollow "pockets". You want to move your wires across. You can do that one of two ways. One is to rip out your drywall and drill holes through the studs. Not much fun. The preferred way is to cut your rectangle hole for your new box. That will give you just enough room to get your hand in the wall. Now, go up to the attic or down to the basement and try to figure out where the hollow pocket is that aligns to your new box. You run a stiff metal tape through the wall and fish it through.
If someone thinks it's fun, you could invite them over to help you. ;-)