Dig a small pit in the ground. Give it a diameter a few inches less than the length of the flat stone. Make sure that you dig in an area free of grass, leaves and other organic debris.
Surround the pit with a layer of rocks. Add another layer or two on top of the first, to make a wall approximately 1-foot high. Make sure that all of the rocks are firmly supported, and do not move or slip when pressure is applied to them. This standard campfire pit will contain the fire.
Lay wood for the fire in the pit. Place small pieces of kindling on the bottom, and larger pieces on top of them.
Lay the flat stone on top of the fire pit. Leave enough of a gap so that you can add more wood to the fire when it is needed. Make sure that the stone is fully supported, and does not rock or slip. The stone needs to be large enough to hold all of the food you intend to cook. You need to heat the stone before cooking on it -- the thinner the stone is, the faster it will heat.