Once you are practicing how to play a slice in tennis, you need to start off with a continental grip. The continental grip can help you come under the ball easily. Grab your racket and simply stand in the middle of your court.
Now whenever your opponent is playing a shot, you must make sure that your racket is high in the air. If your racket is as high as your shoulders, it is perfect for you to play a slice.
Once the ball is played to you, swing the racket down and slice the ball. In other words, you will basically be chopping the ball in such a way that you can add spin to it.
The chop must start from above your shoulders and end above your waist. It depends whether you are playing with your backhand or forehand.
Once you slice the ball you will notice that it will not travel with a lot of speed, and will be extremely low. There are chances that you might hit the net for the first few times, but once it starts crossing the net, it will be hard for your opponent to play an agressive shot.