What are you confused with? If you ever saw a page flip animation where a drawing is done in the corner of a book page on every page and you flip thru them quickly to see the animation, that is pretty much the same thing as what you probably want to start with.
It's pretty easy. You basically just use a keyframe for any frame you want to draw on. The best way is to first do the legs in their furthest opposite positions and then do the frames where the legs are crossing each other. I attached one as an example. It's not great but it illustrates the way to do it. I hope this helps and good luck!