1. Wood

Your degree of success will be higher if your fuel is relatively dry.  It is also helpful if your fuel is segregated and burned by size.  Mixing large pieces of wood with much smaller fuel will result in the large pieces not being completely charred and the small material turning to ash.

2. Preparing the burn site

Before setting up the kiln, clear the area under and around it of burnable material.  A lot of heat will be generated, duff will dry quickly.  Check for roots near the surface that may ignite and smolder underground.

3. Kiln

Find a metal cylinder with a proportional radius to height ratio, such as 4 foot tall by 4 foot wide. The proportions do not need to be exact, as a 55 gallon barrel will also work well. Taller heights with smaller diameter tops will result in cooler burns (below 850 degrees F) with less stable carbon fixed but a higher conversion rate of biomass to charcoal  (Frederik Ronsse, GCB Bioenergy (2013) 5, 104–115).

Steve explains the physics of the cylinder burn in the video below.