Step 1: Have your dog lay down.
Step 2: Lure him with the training stick or with your treat to have him move over to one hip and on his side. Click and treat.
Step 3: Use the lure to get him to go over on his back. (This may take time for some dogs). Click and treat.
Step 4: Continue using the lure to get him to roll all the way over. Jackpot when he does this. (Some people have used gently sloping ground to make it easier for the dog to roll all the way over).
Step 5: When your dog is rolling over more readily, stop using the lure. Expect more before you click and treat, but praise and jackpot when he performs well.
Step 6: After he is rolling over smoothly, start giving him the verbal command "Roll over." A circular hand signal is also helpful. Always praise and intermittently click and treat when he performs well.
Work on the down command before teaching this trick, it will make the first step easier.