First of all you need to have a firm grip over the racket. Then you need to bounce on your heels in order to order to find your rhythm.
You need to adjust the racket high. Remember that you should be looking in a running stance as you will be putting the opposite foot behind.
Next you need to swing the racquet and you can do this by bringing your arm down. You should bring it almost five to six inches down. Then keep it there until the ball is hit back. Remember it should return to the same height on the opposite side.
You need to face your hips in the opposite side and bring your arms over your shoulder. Remember to bring your racket over your should along with your arms.
Hire a professional
You need to hire a coach in order to perfect this technique. The coach will train you how to hit a slice and what angle you should make etc. He will teach you approaches for how to execute this technique. In case you are doing it in wrong way, he will correct you and perfect you.
After learning how to do a slice in tennis, you need to practice it several times a day. Keep on practicing this technique for several days until you get perfect. Only practice will make you to hit a slice perfectly.