The first step is to make a shoe last which is like a dress for for your foot. She shows you two ways to make your lasts. I didn't like either way so I decided to do it the sewers way and make "duct tape doubles" of my feet.
How to cast a model foot from an alginate mould. Note: this is not a proper last as a last is not an exact copy of a foot. But I do think this could be helpful for projects like making greaves. The biggest fault I see here is that he casts without bearing a normal load on the foot which, of course, drastically changes the shape of the foot.