First you need to dig a pit.  The size of the pit depends on how big your pieces are. Build a fire in the pit using kindling and wood. As the fire is heating up, position the dry pots near to the fire to warm them up. When the fire has burned down, but the wood is still hot, spread the pieces of smoldering hot wood out.   Now place the pots on the hot embers. Add more wood around the pottery and build up the fire, so it’s burning around the pots. When the fire around the pots is established, lay pieces of wood across the top of the pottery. As the fire burns down keep adding more wood to keep the fire burning.  Once the fire has been burning for around 5 (!) hours, you can let the flames burn out.  Then gently cover the glowing embers with dirt. Once the heat has gone and the pots are cool enough, you can remove them from the burnt-out embers. When you remove the pots from the fire, they will be black and sooty with carbon from the fire.  They can be cleaned up and polished to a lovely sheen. Source and more information can be found here.