When it comes to sound quality, you're better off having the most power, and the better speakers in the front of the car, but assuming you still want the amp power going to the back, it's really quite simple.
Assuming the amp is already in the car and connected to the head-unit on a full-range source (as in not 'subwoofer' out: Just grab a couple of pairs of speaker wire and go from +/- on each channel output to +/- on each speaker respectively. If applicable, switch to 'hi-pass' output, and adjust to about 60-80Hz, whichever you think sounds better in that range.
Disconnect the speakers from the standard loom first though obviously, and tape them up somewhere safe so they can't short on anything. (Leave them there incase you ever side to sell the car and decide you want to keep the amp)