Lay your baby on her back. Hold the ankle and thigh of one of her legs and gently cross it over her other leg. Watch as baby rolls to the side. It is important to note that this motion should not be forced; it should happen naturally. If your baby is not ready to roll then simple do a different activity and try rolling another day.
Return your baby to her back the try the other leg and rolling toward the other direction. You can sing or say something simple such as “roll” or “(Baby’s name) is rolling.” This is great exercise for your infant and the time interacting with you is key.