To get the feel of how the slider should roll off of your index finger, use this grip and practice at half speed and roughly half the distance to the mound.
Steve CarltonIt (the slider) just rolls off of your index finger and begins it's spin which will take it down and across the plate (hopefully). Just remember not to twist your elbow or wrist. It should be thrown, with the wrist and grip set, just like your fastball, slightly off center - with the same velocity and intensity.
Steve Carlton