This video will show you how to remove ‘cotter pin’ cranks.
Cotter pin cranks are generally older cranks and are found on older bikes.
The seat clamp in the video is used to hold the crank stationary while hammering occurs. The chainring side of the crank will always be a bit more difficult to remove on cotter pin cranks because the chain rings get in the way. This can be solved by using a long piece of metal that extends past the chainring to hammer on or just be persistent.
All you will really need to remove the crank is an adjustable wrench and a hammer. This is a little hex nut on one side of the cotter pin. Remove this hex nut first using the adjustable wrench.
The cotter pin will come out of the cranks from the side opposite the threads, so to remove the pin you will hammer the threaded side. Give it a couple hard whacks. If you are hoping to restore the cranks or use them again be more careful during this step since the cotter pin can bend if it is hit too hard. They do not really make cotter pin cranks anymore, so if the cotter pin is bent you will have to replace with more modern cranks. Hammer on the edge of the cotter pin as square as possible. Once you hammer the cotter pin down it should slide out the other side without much more than a wiggle. However, if really stuck, using a screwdriver or something that will fit in the whole and tapping it out with the hammer will work as well.