Drill or safely shoot air holes into the lower sides of the barrel - about three or four on each side will do. It does not hurt to add more holes up the side (but they are not necessary) unless you add so many the metal drum becomes unstable.
Some folks drill or shoot up to two ½ holes in the bottom of the barrel to create additional air flow. If you choose to do so, this will improve rain drainage, but mandates occasional scooping out of ashes around the base of the burn barrel.
Place two cinder blocks in the desired barrel placement spot.
Sit the 55-gallon drum on top of the cinder blocks.
Use tin snips or a steel saw to cut the hardware cloth and metal sheeting covers to fit - if you are using them.