"The slider was invented to trick the batsman into thinking that the delivery is a normal length spinning ball but instead the ball slides on with the arm. The grip is exactly the same as a normal leg spinner. The key principles which apply to leg spin, apply to the slider.
The only difference is the point of release. Instead of coming around the ball to create spin, try to bowl the ball out of the front of your hand. Spinning the ball off your third finger, creating back spin. The palm of your hand should face towards the batsman, as you can see here. This should create over spin, and the ball will continue on the same path, as opposed to spinning.
The best line and length of the slider is straight at the stumps. This should create chances for Bold or LBWs."