The roll is performed just like the kneeling ukemi. When doing a roll bend over and get as close to the floor as possible, the first thing that should hit the floor is your supporting arm/hand (as in a kneeling roll). Next you tuck your arm (right arm if your are rolling over right shoulder and vice versa) in under the supporting arm, tilt your head, and roll over the shoulder and back. (again just as in the kneeling roll.) The trick, especially when you are starting out is to do it slowly and to try to get as close to the floor as possible before your feet lets go of the floor. I hope it helps