And another method from Stohlman - useful if you're doing a repeated design - is to make your own stamp out of leather. Not something I've tried yet, and only really useful for marking cased leather in preparation for carving, rather than for deep stamping.
Trace the design onto a piece of cased leather (in reverse!), use the swivel knife to make deep cuts, dry the leather rapidly to harden it and open the cuts (e.g. in a warm oven with the door part open), then apply lacquer onto the leather to further harden it.
This can then be laid on the cased leather work piece and tapped gently with a soft mallet to make an imprint for you to use as the basis of your carving, to leave slightly raised lines on the work piece.
I suppose you could do the opposite, and background around the lines on your pattern to leave the design as raised lines on the template, which would then make small indendations when it is used.
Geoff