How to animate squash and stretch in a bouncing arching ball?

I’m doing the Animation Bootcamp Tutorials from CGcookie.com But I’m stuck at the squash and stretch part. There’s no step by step showing how to do it, only a video explaining the concept. The concept is easy enough to understand but the actual implementation not so.

Here’s a screenshot of the animation I’m adding squash and stretch to.


Could someone in a few steps explain how to do this?