I used the method in Tomosoy's book "Repairing Classic Cameras"
Not having illustrations will make it more difficult. Never stopped me before though. Shoe leather may be too thick though, I use leather from old leather purses, dresses or jackets.
1) cut a manila folder or hanging file into strips as deep as you want the cap and tape them around the circumference of the lens. You're going to need 1-2mm thick so it may take a couple of strips to build it up. Cover the strips and the lens with a plastic wrap to protect them from getting wet. Use a rubber band just below the strips to hold the plastic in place.
2) More strips, same size. This gets a little messy. Thin some white glue(Elmer's) or similar, carpenters glue also works. Put it in a small dish or bowl and thin it I use about 50/50. Dampen each strip in the glue, and wrap them around the front edge of the lens, line up the edges as you go so you don't have lumps under the leather.
I put the lens face down on my work area so the edges line up.
Hold the strips in place with rubber bands and let them dry.
3) Using the lens & strips as a guide cut a disc to fit the front of the lens. I use the backing from a note tablet, two pieces glued together +/- 1/16-1/8" thick. and glue the disc to the strips. Let it dry and sand the edges smooth.
4) Turn the lens right side up
I hope you have your leather handy now, it's beginning to look like a lens cap. You're going to need a large enough piece to go over the top and sides, both inside and outside. Won't hurt to have extra.
Coat the outside of the form with white glue(diluted), Wet the leather in water, wring it out and pull the leather over the top and sides. Smooth it out, hold it in place with another rubber band just below the strips and dry it.
The inside is sort of a PITA(to me). You need something for the inside of the disc, leather, whatever, glue it in place, apply glue to the inside edge and push it down on the lens. Leave it in place until it's dry. Some of that shoe leather might make a good lining for the end of the cap too.
TaDa! A Lens Cap!