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This should be done as the ball reaches the highest part of the toss. As your shift your weight to your back foot, lift the ball of your front foot. In this stance, your body is ready to launch off the ground and into the ball. This helps give the serve its power. Your racquet arm should begin in a bent position behind you as you prepare to serve. Your racquet should be straight up, a little higher than your head. As the tossed ball starts to drop, swing your racquet in a whipping motion. Your arm should be completely extended when it comes into contact with the ball. Focus on hitting the ball squarely from behind. You aren't aiming to create any sort of spin— simply try to hit the back of the ball dead on, making only brief contact with the ball. The follow-through of a flat serve is extremely important. Make sure that, after hitting the ball, you continue to swing through until the racquet is at your opposite hip. If you pump the brakes too early, the serve will not have its full force. Transfer your weight so that it is distributed evenly over both feet. Keep your knees bent and shoulder width apart so that you are ready to move. Bring your racquet to the center of your body and hold it with both hands, allowing for quick grip changes and rotations. By staying light on your feet, you will be ready to get anywhere for your opponent’s return.

Summary:
Transfer your weight in your back foot. Swing the racquet arm up and over your opposite shoulder. Bring your racquet down across your body. Prepare for your opponent's return.