If your dog has learned how to lie down and stay, then it's easy to learn to roll over. Let him lie down first, then say "roll" command to him, at the same time, Cling to his chest, let him roll over from one side to another side. At first, you should train him for more times, after that, you may not help him to roll with your hands anymore as soon as possible.

When your dog begs for food, he will pay attention on your fingers and look at the food in your hand, the hand holding food in should be on top of his nose, hold your hand into a bow-shaped, from left to right over his body, then the dog will make a 180-degree roll following the food, as well, you should put one hand on his body to avoid him to jump up.