1. A metal can with lid (I use a paint can)
2. limbs from desired tree
3. a good fire
4. axe, knife, nail
Make your charcoal oven by punching holes in the lid of your metal can with the nail. I used more holes than I needed but about 6 should do it if they are fairly large nails like 6 penny. If you use a paint can like I do remember to thoroughly clean it out and set it upside down on the fire for a few minutes to burn out the remaining paint.
Cut limbs from the tree you desire. Now to prepare it for the oven cut them into logs small enough to fit in the can. Strip all the bark off of them as the bark contains dirt and stuff that will make ash and be bad for BP.
Now simply fill your can up, put the lid on and set "in" the fire. Place it right in the hottest point. After a few minutes it should begin releasing steam as the moisture is baked out of the sticks. When this has happened for a while it will begin realeasing volatile gases which will probably ignite if you have a good fire. It doesn't have to ignite I just like it to since it makes a good timer! When the fire on top goes out on top you know its done. However if it doesn't ignite just wait till all visible gases stop then give it a few more minutes.
When its done just take it off the fire and let the can cool before taking the charcoal out. The charcoal should look like the sticks but black and very soft. Grind these in a ball mill or mortar and pestle to be usable in BP.