How do you teach a guinea pig to roll over?

You'd need to use the same technique used to train a ferret to do anything else. When you play with the ferret, watch carefully to see when it's about to roll over as part of its natural play and when it's about to do so, use a vocal command such as simply saying "roll over!" If it does so, reward the ferret with something it likes, such as ferret treats. If you keep this up, the ferret will learn that it gets a reward for rolling over when it hears you say those Words and will do it on command, so you'll no longer need to give it the treat.

Ferrets are more like cats than dogs in many ways, so do not expect it to do exactly what you ask every time. Whereas a well-trained dog will do what it's told every time it is told, a cat or a ferret will only do so when and if it suits them.