As one who has done a fair bit of leather work the above are about right... (my wife and I both carve and stamp leather and We've made quite a few cases, eye patches, journal covers and so on).
First point. You need to be using Vegetable tanned leather, not Chrome tanned leather. If you cut into it and the middle is a kind of grey then it's chrome tanned. Whether it's "treated" or not is not so important as long as it's not water proof.
Then you want to get it wet. Wipe it with a damp cloth until the surface colour changes you probably want to play with a practice piece for a bit before doing the final design so that you have some idea how wet you want it.
Then you add your design... If you don't want to buy specialised tools then you can use a dull point (knitting needle, rounded nail, empty ball point pen) to mark your design. You can press harder to start stamping the design in. Al Stohlman originally taught himself using a knife, some "spoons" and stamps carved from nails. He didn't case the leather (make it wet) so it was much harder... But it is possible like that.
If you have a fancier design you might want to "carve the design" and then stamp it. The simplest method would be to cut the outline (no more than half way through the leather) with a very sharp knife (or a "swivel knife" if you want the proper tools) and then use a small flat punch to press the design down (your choice if it's inside or outside, both are normal techniques).
For proper tools and the right sort of leather you could try Tandy leather factory or your local craft store, but this is one of those places where it can get expensive quickly. If you have a Tandy nearby and you only need a small piece then check out their scrap bins. They also sell dies, rivets, snaps, and fancy conchos (larger metal ornaments) and studs.
I hope that this helps,
Z.
