There are two basic options for a 55-gallon drum barbecue smoker: horizontal or vertical. Either will work. Horizontal smokers split the barrel in half lengthwise, use the top half for a lid and install some centre baffle to separate the firebox from the grate in the smoking area and direct heat and smoke to the meat. Vertical barrel smokers typically have a grate on top, a firebox at the bottom and a water pan in between. These rely on the natural upward flow of heated air and smoke to cook and flavour the meat. Vertical smokers are easier to build.
Clean the drum thoroughly (and carefully) to remove all traces of any petroleum products. Remove the drum top. Scrub the inside with strong detergent and a wire brush until it is perfectly clean. Some builders build a fire inside to eliminate any remaining oil. Clean the outside, too, so the barrel is almost like new.
Turn the barrel over so the open end is down. Cut the top fourth of the barrel off, with a metal saw or welder's cutting torch, and weld a handle on the top, to make a top cover for the smoker. Weld four small steel brackets on the bottom of the cover so it will sit on the drum (these can be cut and fashioned from the metal of the top). Cut a door opening near the bottom of one side of the barrel to provide access to a firebox. Weld hinges, a twist latch and a handle on the cut out part to make the door.
Weld three brackets just above the bottom of the barrel, three more about halfway up and three at the top. The bottom brackets will support a firebox, the centre ones a water pan and the top ones a grill. Form the brackets as simple triangles, using part of the drum top or scrap steel; they need to be just long enough to fit the lip of a pan.
Buy a heavy steel pan, like a wash pan, for the firebox and a second, smaller, pan for the water pan. Size the pans so they can fit through the top of the barrel to be placed on the brackets. Buy a grate to fit the top opening or cut one out of expanded metal flooring. Add a thermometer to the lid; these can be bought at building supply or barbecue supply store and typically just slip into a hole cut in the lid.
Don't try to build your own smoker unless you have welding skills.
Work very carefully when welding on any material which was exposed to flammable petroleum products.