It is important that your dog knows the 'Lay' position in order to easily teach this trick.
At you command, the dog will roll over his back (you can teach him to do this even more than once)
How to do it
Starting from the 'Lay' position, say 'Roll Over' and "roll" gently your dog (grab his legs and pull him, or push him from one side so that he makes a complete roll). After he has completed the roll, make him sit, cheer him and give him the treat.
Tips & Tricks
This trick is a quite hard one: your dog will have to lay with his belly up, and not many dogs like this position (it means "I surrender completely to you"...). So if your dog doesn't like to roll over, don't insist too much: there are plenty of other tricks you can teach him! :-)