The first step and most important is to thoroughly clean the 55 gallon drum to remove any residue from the inside.
Carefully cut the 55 gallon drum in half. This can be done with a reciprocating saw. Once your cut is complete, grind down the edges of the cut to remove burrs.
Build the supporting frame for the drum. It should be sturdy and welded, not bolted.
Measure the inside of the bottom half of the drum and using steel mesh construct a grate. Weld the grate to the frame such that the charcoal will not fall through. It should be a couple of inches off the bottom to allow for air flow.
Using more grate or mesh, construct a surface for your food. It should be 6 - 9 inches above the grate for the charcoal. It can be removable for cleaning purposes, so use bolts drilled through the side of the bottom of the drum to support it.
Hinges may be bolted or welded on. They should be at least 4" long, butt hinges, and should be fixed pin for safety purposes. Use three along the seam of the drum halves.
Finally, drill holes in the side of the bottom of your cooker. Several half inch holes placed in a 8" circle will perform well. Using a round 8" plate, drill and bolt it through to act as an adjustable damper. Drill the same hole pattern in the opposite end of the top half of the grill.