The purpose of a slice is to keep the ball low so your opponent does not have a good chance at a passing shot!  When slicing on the forehand side, the ball is hit while moving in towards the net with a continental grip.  By using a continental grip it opens up the face of the racquet making it easier to slice the ball.  Your body should be in a semi-open stance off the back foot.  When finishing the shot, the racquet should be pointed in the direction of the balls path.