Cotter pins work by preventing the bolt in which they are inserted from coming out. To use a cotter pin, the bolt will need a pre-drilled hole in which to insert the pin.If you drill it yourself, you can materially alter the structural stability and strength of the bolt. It is better to buy a bolt that is designed specifically for use with a cotter pin.

To use a cotter pin, simply insert it into the bolt’s hole. Then you pry each leg outward so they are at an angle. Some may claim it's best to twist the ends of the pin together, but this is not accurate.