1. First add a scale key (Shift + R) to the first hit key. This will prevent the ball from squashing before it hits the first contact.
2. In the timeline, copy the first hit key and paste it into the next frame. Here's how! In the timeline, Right-click the first key, choose Copy. Right-click the next frame, and choose Paste > Paste.
3. Using scale (R key), squash the ball on the squash frame. BE SURE TO CHECK THE SQUASH IN SEVERAL WINDOWS otherwise, you're going to screw up the ball's volume. Bad. Scrub the hit and squash several times to make sure your ball isn't shrinking or expanding on the squash. Scrub it in several windows. Helpful hint: Holding down control and moving the scale manipulator on one axis will freeze scale on that axis and scale along the other 2 axes.
4. Now you need to tell the ball where to return to it's original scale. I'll be boring, and choose the apex fame after the first contact. I move to that key in the timeline, open up the channel box, and set each of the x,y, and z scale inputs to 1. Make sure, you're in the SCALE boxes, and not the rotation or transform boxes. Easy to mix those up. Playback your animation and save it using .0X suffix.