Give plenty plenty of opportunities for rolling about during awake time. Lots of floor time, and tummy time, especially tummy to back activities ( set up a little gym, put her on her belly under it to encourage the rolling to her back.
After you put her down, and she's into her sleep. Quietly and discreetly as humanly possible, go in prior to her "roll over" habit. When she goes for the roll, put you hand on her to stop the roll. Act as though you are not even there.. no talking, or trying to engage her attention. Keeping stopping the roll, by blocking it with your hand on her hip and shoulder.
Still let her shift about.. just block the roll. Also, make the room as dark as possible so she doesn't "see" you.
What you are teaching her is awake time is for rolling around, nap and night sleep is for sleeping.