I hope it's okay to just post a link, I didn't want to spam the thread with pictures.
I've come up with a simple way to make a basic leather boot and written it up in tutorial form over at my dA.
It covers how to make a boot that laces tightly to the leg, but I'm sure there are ways it can be modified for making a boot that stands on it's own. This is for those times when a boot cover just isn't enough!
This was used to make boots for Sokka and Zuko. I... haven't actually been able to pin Sokka down to get pictures of his, but Zuko's were used and pictured in the tutorial.