Unless you've invested years of practice, over-the-shoulder throws and flips usually aren't the best way of defending yourself against a bigger opponent. Most over-the-shoulder throws require you to get your body very close to your opponents -- the exact opposite of your ultimate goal during a self-defense situation. Still, you can still practice a basic over-the-shoulder throw for in preparation for a martial arts tournament, or to build strength, body confidence and a better understanding of leverage.
Stand facing your opponent in a comfortable, relaxed stance, feet hip width apart and arms by your side. Once you feel comfortable with the basic technique of the throw, you can practice doing it from various positions -- for example from a clinch or while defending against a strike.
Grasp your opponent's left hand with your right, and pull him slightly toward you. Getting him off-balance now makes him much easier to throw. You could also arrive at this position from a clinch or while defending against a strike.
Step forward and right with your left foot at the same time as you pull, then pivot on that foot, swinging your body so that you face roughly the same direction as your opponent and your hips are very close to his. The key to performing a clean, effective throw on someone bigger than you is getting your center of balance -- for general purposes, consider that your hips -- directly below his.
Swing your left arm around your opponent's torso if possible. You'll use it to guide the position of his upper body, but not to force it. Squat slightly, to get your hips below your opponent's, and continue pulling on your opponent's left hand with your right. His hips should end up over your hips and his left armpit should end up over your left shoulder.
Straighten your legs, lifting your hips -- and thus your opponent's -- straight up. At the same time pull his left arm down and to the right, as if you were pulling a towel over your left shoulder. If you combine these two movements properly, he will go flying over your left shoulder.