If you want to mimic the design of the passive composter where you add things to the top and harves from a door on the bottom, then you would cut the whole bottom off to have it exposed to the soil so the microbes can work their way up. you would have some kind of door near the bottom of the barell to collect the compost and a lid at the top to put the compost in. If you are going to build this kind of composter. I would put composting worms in it . Carbons and greens need to be chopped fairly fine or they will take very long time.
Something similar to this one.
The commercial composter is 90 gallons so twice the size of your 55 gallon drum. It might be better to make the composter out of a 90 gallon trash can. It already has a lid you just have to cut off the bottom to get it on the ground and make a door for it on the bottom.
I'd save the drum for a water barrel, cut it lengthwise for planters, cut the bottom and top off and use it for a planter for gobo or parsnips which have really long roots, or cut it in half and get two ice buckets for a backyard party or half barrel planters. You could also convert it into a tower planter.