The treadle can be as simple as a stout stick, indeed, I have seen a bodger use just this when demonstrating. Not because of any misguided machismo but because he left his at home by mistake. A better idea is to make one that is comfortable to use over a long period. Contrary to what you might expect it is not the leg doing the treadling that starts to ache first. It is just as important to make sure the foot you stand on has a flat surface on which to rest. The back foot also serves to anchor the position of the treadle. To this end the treadle is attached to a board about 18"(45cm) wide by 15"(40cm) front to back. My favoured treadle has a board made from 6mm plywood, thick enough to be robust but not so thick as to present an uncomfortable ridge if my foot is partly over the edge. The treadle itself need only be made of softwood. I use parts of an old leather belt to make the hinges. Metal hinges will tend to buckle on uneven ground whereas leather copes well. The treadle needs to be about 3ft (1m) long. If it is longer it will have a longer stroke but the cord will be at an acute angle to the work.
A rounded end to the bottom of the treadle will help prevent the treadle creeping forward as you work. The treadle ends should rest on the ground and not on the board, so make sure there is enough slack in the hinges to allow this. Attach the cord to the treadle head by a simple double half hitch to form a slip noose around the head. To shorten your cord to the desired length, just wind it onto the head a few turns. Round off the top of the treadle bar so that it it comfortable for your foot when pressing down on the treadle. To work the treadle, stand with one foot on the board and press down on the treadle bar and release it. A nice steady rate of about once a second is fine.