Patrice,
Actually the process works very well. I started playing around with doing this about a year ago for making some leather versions of charms I have cast out of silver and gold.
What I like to do is first make a wax piece using the mold, when cooled I remove the wax from the mold and make the necessary trimmings on the wax.
Next I soak a piece of leather (1/2oz) in warm water. Lay the leather in the mold and place the wax piece in the crevass of the mold. Now I simply use pressure to make the impression on the leather. I use my clamp type vulcanizer, but a couple blocks of wood and some "c" clamps work just as well. Just leave it there for an hour. Take the leather and wax pieces out together and see how good of an impression you have. You can fill in the details by spraying a little water on the positive side of the impression and using a modeling spoon. When it is where you want it, stop, and spray with water until saturated, and let it sit overnight.
Now that it is dry you can do your coloring. At this point, take out the wax piece.
Finally you have a couple options.
1. Coat the wax piece in a few coats of a clear acrylic finish. and glue this in your leather piece as a plug.
2. use sawdust or leather dust to fill the void.
toss another piece of leather on the back and glue it down. trim and glue the edges.
and wallah you have a finished piece.