The first step is to get your dog in the right position. Have your dog sit or lie down, and then place a treat in front of their nose.
Slowly move the treat over your dog's head and towards their shoulder.
Soon as your dog rolls over, mark the behavior with a clear and consistent sound, such as a click or a "yes!".
Repeat steps 2 and 3 several times, until your dog is reliably rolling over when you lure them with the treat.
Once your dog is reliably rolling over, you can start to fade the lure.
It takes time and patience to train a dog to roll over.
Training your dog to roll over should be a fun experience for both of you.