Google gave me this:
**broken link removed**
I have used it, works fine up to about 20v, and 500mA, the uln2803A chip can run up to 40v and 750mA if i'm not mistaken, it's normal ro get hot.
so instead of the serial port wires, hook up 4 pins of your micro, and make them go on and off in a sequence, the faster the sequencing, the faster it spins, obviously.
stepper motors will drain your batteries really fast though, the best option would be dc motors with gearbox and encoders.
oh, and don't hook up 4 wired steppers to that without changing the sequencing code, or your chip will burn.