Start with a boot that has the right heel and toe shape. Here we're going to be altering the leg shaft but leaving the foot intact.
We've cut the leg shaft off, but carefully left the zipper intact. The zipper will be used later! The pink strip is a strip of leather that has been glued to the inside and will be used to secure the new leg shaft. Make sure to glue the leather rough-side to rough-side for better adhesion.
Making a pattern for the new leg shaft out of muslin.
Here we have the muslin pattern taped to the boot to test it out.
Once you have a well-fitting muslin, it's time to use leather! The colour doesn't matter so much as you'll be covering it with paint, but you can try to find a close-ish match just in case the paint gets scratched.
Here we have cut the leather and hemmed it using a leather needle on a standard sewing machine.
The new leg shaft glued in place. We used a strip of leather about 1" wide all around the opening, half glued to the bottom part, half to the top.
In the second photo we've glued the tails of the zipper to the new shaft. Here the shaft extends further, but the leather was stiff enough to stay in place. If needed, you could add some velcro to the extension.
Make sure to use a flexible glue designed for leather, such as E-6000 or Barge.