Here is how to make it, taken from (from Traditional Woodland Crafts - Raymond Tabor. Batsford Books 1994).

This simple process requires no capital, takes very little time, can be easily stopped, and requires minimal supervision. It is ideal for anyone with small amounts of wood waste not easily saleable as firewood.
1. Using a cold chisel prepare the drum by making five 50mm 92in) holes in one end, and completely removing the other. Knock up the cut edge of the open end to form a ledge.
2. Position the drum, open end upwards, on three bricks to allow an air flow to the five holes in the base.
3. Place paper, kindling and brown ends (incompletely charred butts from the last burn) inot the bottom of the drum, and light.
4. Once it is burining well, load branchwood, at random to allowair spaces, until the drum is completely full. keep the pieces to a fairly even diameter, but put any larger ones towards the bottom where they will be subject to longer charing.
5. When the fire is hot and clearly will not go out, restrict air access around the base by using soil piled against it, but leaving one 100mm (4in) gap. Also place the lid on top, lving a small gap at one side for the smoke to exit.
6. Dense white wet smoke will issue during the charring process. When this visibly slows, bang the drum to settle the wood down, creating more white smoke.
7. When the smoke turns from white (mainly water being driven off) to thin blue (charcoal starting to burn), stop the burn by firstly closing off all air access to the base using more earth, and secondly placing the top lid firmly on its ledge, making it airtight by the addition of sods and soil as required. The burn will take between three and four hours.
8. After cooling for about 24 hours the drum c be tipped over, the charcoal emptied onto a sheet, and graded and packed.