Although slower to execute than its cousin, cut kick (front leg side kick), the rear leg side kick can be very powerful and effective when done properly with speed.
Here are the basic steps of the Side Kick laid out in a horizontal fashion as a quick reference guide. You may get the poster size of this by clicking on the picture.
Now that you have the basic steps, you must practice them and refine your technique. Get those motor skills programmed in and fine tune the mechanics. As I said before, I would work with an instructor on the details because there are a lot of details in between each step and far beyond the scope of this guide.