The best way to teach your dog to roll over, is to shape the behaviour. Shaping is when you gradually build up the desired behaviour by rewarding each step the dog makes that brings him closer to your target, which, in this case, is to roll over.
Get your pooch into a sitting position. Reward.
Get your pooch to drop. Reward.
With the food, lure your dog’s head over his stomach, ensuring your hand is slightly above the centre point of your dog’s back. When he lies on his side and has his head reaching for the food, reward.
Gradually begin moving the food treat further towards the other side of your pooch’s body. When he rolls onto his back, reward.
Now that your pooch is lying on his back, move the food away from the pooch, and away from the side of the dog you originally started from. This will cause him to roll over completely. Reward and praise!