Obtain a strong, well-constructed barrel. This barrel must have minimal rust or other blemishes or damage that will weaken the structure of the barrel. Check each side of the barrel, including the bottom. A 55-gallon drum will typically meet your burning needs.
Cut holes in the barrel to allow oxygen in. These holes should be about 3-inches square. Put three near the bottom, spaced evenly around the barrel. You can place more vents around the middle of the barrel.
Cut screens out of five-eighth inch woven metal mesh for each vent. Screw the screens over each hole to prevent sparks from flying through the vents and creating an unplanned fire.
Lay a similar piece of mesh over the top of the barrel while burning. You can remove it long enough to put more trash in.
Place the barrel in a clearing where no grass or other flammable materials are within 10 feet in any direction. This will minimize the chance of an accidental fire.