Start out with a simple, small flat Lego block with four small wheels.
Then cut a foam tray down the middle and taped the two sides together so that it was the size I wanted it to be. I did that because I wanted my battery and power module to stay on it.
Tape the small wheels on the flat block to the underside of my tray. I used two DC motors and attached two big Lego wheels and used a wire to allow them to be side by side.
Attach the big wheels onto the car by taping the DC motors to the underside of the tray.
Turn on the power module and it rolls!