I would be a little wary of smoking directly over fire. You don't want the heat from the fire, just the smoke. I have seen people you cheap ducting and cardboard boxes to direct smoke over their grains.
Basically, you need to create a fire in an enclosed space, direct the smoke out of the enclosed space (cheap ducting), and into the cardboard box where your grains are sitting. You could try the camp fire, but try to keep the grains cool (relatively).