In a place far enough away from flammable structures and trees (including tree roots), a place that is sheltered from strong gusts of wind, dig a shallow pit (you only need to go a few inches deep) a few feet larger than you want your fire to be (if you can, it is best to dig a bit deeper and then fill the pit back up partways with sand). Encircle the perimeter of the pit with rocks or bricks. Have buckets of water, sand, a fire extinguisher, or what not, on hand at all times for safety and to put out the fire when it's all over.