Step by step could be tough, but to start you'll need some wood, hinges and some sort of bracket for a lock. Id suggest finding a piece at least 1.5" thick, and preferably wide enough for you to make the whole thing out of that one piece for a more natural fit. Once youve cut it in half figure an appropriate length for the footholes, 12" apart should do. Once the boards in half you can cut the footholes, itll be easier to cut them out since the edges will be exposed, i recommend drawing where you want that cut before cutting the board in half so the holes line up. For width of the whole try making it big enough for an ankle to fit comfortably but not so big a whole foot can escape. Toe ties csn be done with simple a good nail sticking out of the top, but that can wait until last really. After all those cuts are made get the hinges on, the. You can do te bracket for your lock. I recommend lining the ankle holes with something soft, anything from leather to sweatshirt thick cotton, or you can just sand it really smooth. That should cover everything but a stand really
