You don't need disc sliders for this exercise; just grab a pair of towels and get to work.
Begin in plank position with two towels under the balls of your feet (or paper plates if you're on carpet).
Slowly bend both knees in toward the chest while keeping the arms straight, shoulders over the wrists, and maintaining a flat, tabletop back. If this is too difficult, draw one knee in at a time.
This counts as one rep.
For a variation, draw both knees to the right foot, straighten the legs back behind you, then draw both knees to the left foot.