I've never done it either but if I had to I would cut a piece of leather bigger than I need, you could use a piece of paper or material to get the rough size then soak the leather, put the leather over the boot toe, put a piece of foam over that then tape it tight over the boot.
You might have to work it around the boot a bit before putting the foam on.
When you remove the foam after waiting a day the form will be in the leather and you will have to trim it to fit.