Punches are made out of strong metal so that you can hit them with a lot of force. You can mark light metal (like the following hobby material) and hard metal. (like keys) They come in different sizes and fonts, these don't include numbers but some do.
To fight rust, they're sometimes conditioned with oil. If that's the case, careful about what you handle while working with them and wash your hands after. (Oil makes for terrible painting surfaces, wash with dish soap before priming if you get minis dirty)
(Why can't measurements be real?)
The surface you're punching against should also be strong and able to take this force as well as relatively smooth. (A rough surface will impart it's texture on your metal.) A block of wood's a good choice. I use a glass palette atop my painting area - you can bet I remember to remove it before I start stamping.
I've cut some paper in the size that I need the wording to fit on. Then, I use a knife to mark the lines I'll be cutting out later as a guide.
This'll take some practice, but spacing and alignment can be tricky. Get the letters as close as possible to each other without overlapping. As you can see on the left, I dropped my "Y" a little.
Punches are usually square so you know which way things are angled. Try to keep your characters straight, unlike the "E" to the side.
Most punch sets are designed to be read from the side you're punching them on. That means the text is sunk into the surface and reads left to right. If your material is thinner, the letters will jut out the other side and be raised. They won't be readable though, because they'll be backwards. (There are probably mirrored punch sets out there though.)
Here's the trick - depending on the letters you use, you can write backwards (right to left) so the letters jut out the other side of thin metal. Interestingly enough, a lot of our alphabet works reversed when using capitals.
Symmetrical: A H I M O T U V W X Y 1 8
Work turned around 180 degrees: B C D E K N S Z 3 5
Can't be turned around: F G J L P Q R 1 2 4 5 6 7 9
Symmetrical letters are the same on both sides if you drew a line down the middle. Other letters can be turned around to face the other direction and still be the same. As you can see, numbers don't reverse as well. (Some numbers might work either way depending on their font, so I've listed them twice.)
After you've got your metal punched, cut it out with a knife if it's thin enough, or use tin snips to cut thicker metal.
And here we have license plates for the Postapocalyptibuggy. "KTHXBYE" - Kind of a modern take on "Eat my dirt!"