Imagine a small square on the ground and you are standing in the middle of it.  As fast as you can jump out of the box and back to starting working your way all around the box.  Then repeat with your left and right feet.

So it looks like this:
Back- Foreword

Then repeat in the opposite direction

Repeat the pattern again with your left and right feet seperately.

Jumping the pattern with both feet and each foot individualy = 1 rep