i'd go buy one and stuff it with cake lol sorry i couldn't help it, i'm not that good. be sure and post a pic
I've never done anything like this, but in theory what i would do is:
build the stand to look like the sole/heel. then on top of that build the show out of cake....put some for the feet part and then build up the boot up the leg. then put dowels for support and cover in fondant.
(that is if the whole shoe is supposed to be cake.
if not, just follow directions for a "regular shoe, and build the boot part with gumpaste putting a cardboard cilinder inside for suppor till it dries..
that's my best idea but i've never done it. i'm sure somebody else has to have a better one!