My technique (discovered in the early '70s) is simple: thread on the cotter nut several turns (but don't tighten down fully) after placing one or two washers on the threaded portion of the cotter. The objective is to leave about 1/8" of space (maximum, about 1/4") for the movement of the cotter. That way, when you hammer it, you can't mushroom the threaded end and it can't bend sideways. All of the force goes straight down.
Then, hammer on the cotter nut. Best to support the underside of the crank around the unthreaded end of the cotter. I used to stand the bike on the floor and place a piece of wood under the crank that had a V cut into it that left space for movement of the cotter, with the other end of the piece of wood on the ground. I also put a sheet of cardboard between the crank and the bike's frame, so I wouldn't have to worry about gouging the frame and could therefore smack the cotter with the full force needed.