How do you step in for a leading rear cross punch? Do you leave the rear foot behind?

[OMGWTF]

You should be doing drills, like these:
1) slide your front foot forward and jab, a slight push off your rear foot. When your back foot slides up, throw that hand. Circle to your right by sliding your right foot to the right, jabbing as you do so, then pivot on it. As soon as your left foot lands, throw the straight left. To circle left, slide your right foot barely to the left (and jab) and, as you pivot, throw the left hand. (when the left foot lands, hook with your right hand).
2) To go backwards, push off your right foot and jab. When you bring your right foot back, throw the straight left. Do this a couple times and you'll get the feel of when you should be punching.
You always bring that rear foot with you; you'll find it much easier to do things once you get where you are going, whether it is punch or duck or get out, you need your feet under you. If you leave it behind, your first move has to be straight back, so you can get your feet under you, and that leaves you vulnerable.
Don't drag your feet. Push, a bit. That bit of a pulse makes a big difference in how you move.

Thanks a million friend your instructions work a treat!!!