Demi plie in open fifth position. Put your left leg in front with the toes of your left foot pointing towards the left. The toes of your right foot should face the right. Your left toes and heel of your right foot will form a line from front to back as will your left heel and right toes. Place your left arm out to your side at slightly less than shoulder height. Put your right arm directly in front of your body, imagining that you're holding a giant ball.
Begin a pirouette while bringing both arms in front of your body, outstretched as though you're holding a giant ball. Lower your right leg in front of you as you turn towards the front left corner while bending your left knee slightly. All your weight will be on your left leg.
Lift your left leg in a releve while you rotate your right leg out to the side. Move your right arm at the same pace as your right leg. Your upper body will be facing forward now, and your right arm and right leg will reach directly to your side at the same moment.
Tuck your right leg in as you turn towards the front, forming a number "4" with your bent right leg and straight left leg. Lower your right leg again as you reach the front left corner. You've now completed one fouette.
Repeat Steps 3 and 4 as many times as you can in a row without bobbling or losing your balance. With practice, you'll be able to work towards multiple fouettes in a row.