In my old Aikido club we used to have people make a circle with their arms as if they were wrapping them around an exercise ball with the palms facing away. If you want to roll over right shoulder, you'd make the circle with your right elbow pointing up and right-hand fingers pointing down. If you want to roll over your left should, you'd make a circle with your left elbow pointing up and left-hand fingers pointing down. Remember that the palms should be turned away from you.
Begin training slowly by adopting the arm circle position and bending forward until your palms touch the ground. Once on the ground, the hand of the shoulder you want to roll over should have fingers pointing behind you. Tuck your head to the opposite side, away from the leading shoulder, as you roll. Contact with the ground should travel diagonally across your back as you roll, beginning with your leading shoulder. As you come out of the roll your feet should be turned sideways a bit so that you don't slam down on your heels.
After practicing this two-arm roll for a while you can begin rolling using just one arm, the arm of the leading shoulder, which as you initiate the roll should have elbow up, palm pointing away and fingers pointing down. But using two arms allows you to come in a steeper angle without collapsing the arms, and is a bit safer.