Both my dogs learned how to roll by using a dog treat and guiding them through the rolling movements. To make it easier, teach your dog how to lay down on command first. Once laying down, have your dog's attention on a treat and then move the treat sideways causing the dog to do a half roll. Say ''roll over'' repeatedly as the dog rolls sideways and give the treat right after the dog has rolled to the side.
It does not happen over night, but most dogs eventually learn in a few days.
Once the dog rolls half way on command, teach to complete the roll completely by moving the treat further.