It's easiest to break it down into steps, using a lure.
Start by teaching the dog to lie on her side from a down. Put a treat in front of her nose and then move it slowly down her flank and then up a bit. She'll follow it and in doing so will end up on her side. Give her the treat. This will take some practice to get the luring motion just right.
Once you have that down, when the dog is on her side encourage her to roll over by luring her nose from one side to the other.
Put it together, and then put a word on it, like "Over!" You can also abstract the hand motion gradually into a finger-swirl or something like that.
We worked on a solid 'down' command first. Once that was good to go, we would take the treat by her nose and around the back side of her head, she will want to follow with her eyes but theres only two ways to still see it. 1) get up 2) roll on her side/back. Once you stop the getting up action and your dog goes to his/her side, proceed with the treat behind her head and she will follow causing a roll over. Give the treat and praise. Do this continually for 30 mins or so, and your dog will have a good grasp of what your trying to do...Do this over 2-3 days, you should have a solid roll over...