Traditional method: Teach your dog "down" and then physically roll them over or lure them over with a treat.

Alternative method: break it down into smaller steps: First "down", then "over" (To teach "over", say "over", hold a treat to the left or right of the dog, and gently shove on the opposite shoulder and urge the dog to roll onto his or her side and relax. Treat immediately and scratch the shoulder.) Next while your dog is in the "over"" position and say "on your back" while at the same time rubbing their belly. As the leg lifts to allow access to the belly roll the dog *barely* onto her back and immediately give a treat and praise her saying "Good back, good back". As this trick progresses, he should immediately roll onto his back expecting a belly rub. Slowly, slowly, slowly phase out the belly rub and substitute a small treat. Finally finish the "roll over" by holding a treat in front of her face while she is upside down, and move it to the side you'd like her to roll up on. The command for this movement is the actual "Roll Over" You can go through all of the steps over and over until the dog anticipates the next one coming , then start skipping commands until you get the whole trick out of just "roll over".