[Alan Anderson]

You want to control your wheel speed. That means you'll need a way to measure the wheel speed and adjust motor power to make the wheels move at the rate you want.

For a quick demo, you might be able to get away with a "bang-bang" controller that simply turns on wheel power when the speed is too low, and turns it off when the rate is too high. For best results, you'll want some form of PID control (look it up, there are a lot of good explanations out there).