Spinning back kick is a great way to keep something behind you. When done right, it is possibly the most powerful kick in the Taekwondo arsenal.
Here are the basic steps of the Spinning Back 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.
Ever try to back kick a round object... Like a pumpkin? I did. Watch me use a variation of the back kick called jump back kick to try and squash a pumpkin: