Hi,
Just to add to what Hawke said about putting your right arm straight out...
At the same time, put your left hand, palm outward (to your right), by the right side of your face. Now, turn your head to the right and drop your chin toward your right shoulder, and just let your weight drop straight down through your hips until your butt hits the ground softly. As soon as that happens, throw your right leg, knee straight, straight over your right shoulder, and keep it going over until the ball of your right foot meets the mat. Then, bend your knee and let it drop until it meets the mat softly (this prevents your knee from hitting first, which would be bad on a real surface). Using the momentum from having kicked your right leg over your head and shoulder, swing your left leg through straight forward (underneath yourself) until your left foot is directly in front of you. Put your left foot on the ground in front of yourself. You should be kneeling on your right knee, and your left knee should be bent at a 90 degree angle. Bring your hands up to guard yourself. Some styles call this a three-point stance.
People who have stiff lower backs or hamstrings usually have trouble with backward rolls. But, if you can touch your toes without bending your knees, you shouldn't have any problems due to a lack of flexibility.
Please note that I do not practice Ninjutsu, so this type of backward roll comes from another style. It's likely that your teacher will show you a way that's at least slightly different. But, the principles are probably going to be very similar. As others have suggested, follow your teacher's instructions.
btw, the suggestion about having sombeody push you down from in front is a very good one. That's one very typical situation in which a backward roll is very appropriate. For forward rolls, have somebody come up behind you and push you from behind.
Best of luck to you...