You start like a kneeling roll, but then you make yourself very small, like a ball, and instead of making a circle with the hand over which you roll, you put it between your legs and reach out far behind your feet. Your head looks into that direction, and once you reach out really far you loose balance and roll directly over your shoulder. Shoulder is already on the mat, so it doesn't hurt. And since all body parts are practically on the floor, this roll doesn't frighten beginners.
I think that quite a few people have the problem with looking in the wrong direction. They try so hard to get down low, put the hand on the mat, unbendable arm, et cetera...but their head and shoulders are still up and facing forward. You're never going to do a safe forward roll while you're looking forward; you just can't get your head out of the way properly and get the roll going over the back of the shoulder and diagonally across the back, and every roll is going to be a scary and bumpy and painful experience, no matter how hard you try. I combine the posture Eva describes, plus the advice to "reach between your feet and look between your feet", and that seems to do the trick most of the time.