I've done a bit of my own. I would say the biggest thing is lots of practice. You'll need to learn just how to position your stamps, spacing, and the pressure to hammer it with. You should get some cheaper materials to practice on before trying to stamp on something you'd want to keep and wear.
Etsy is a good place to find blanks to practice on. I can't remember where exactly I got my metal stamps, but if you do a web search for them, you can find several sets. There are some inexpensive sets that are just letters and basic symbols. Etsy would be a good place to find more ornate stamps like the baby feet in the example you linked.
Good luck!