First you must gather together enough rather large rocks to lay out in this fashion. Try to get them all as close to the same size as possible.

 You keep a coal making fire going in the middle and you rake the coals into the squared off area to cook over. Lay a wire rack from an oven on the rocks over the coals and you are ready to go! Be sure to use a rack from an oven because they stand up to the heat much better than a rack from a fridge, which will tend to warp in the heat from the coals.
 You can arrange the rocks to fit your needs, I always have another ring of stones off the side to do my dutch oven cooking. This keeps the coals contained in one area so you do not have to worry about someone stepping in them when walking around the fire. You can see what I mean in the photo that is linked below.

 
Here is a photo of the keyhole fire pit in action
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