Go to Anvilfire.com, go to navigate site drop down menu. Go to touchmark registry, then to "how to make a touchmark"

Touchmarks are just steel stamps.
They are ground or milled, then heat treated for hardness.
You could use a piece of coil car spring, heat and anneal it, straighten it, and file, grind, or even chisel your mark in it, then harden and temper it.

The very complicated ones are made on CNC milling machines, then heat treated.
Simple ones, like the ones Joe Rollings makes, are ground freehand.
Joe is a master at this- he makes a line of silversmith stamps freehand, quickly and very well, that he sells thousands of. But the idea is the same.
http://www.thingswestern.com/6.html

You start with a decent steel that can be hardened. You anneal it to make it soft. You grind your shape (backwards, of course) into it, then harden it.