This is a simple drill, but it can take lots of practice to master. The player starts with the ball between their legs, and using the inside of their feet, simply taps the ball back and forth. The image is similar to a bell being rung side to side.
With new players, have them start this one slow. Young players will likely keep both feet on the ground for the most part. As players advance, the taps should get faster and faster, and a slight "jumping" motion will soon ensue. When really strong players do this at a very fast pace, it's a sight to see!
For added variety, have the players move the ball (and their bodies) forward as they practice this drill. Then try backwards.