The process of hand-stamping is an ancient method of creating an impression in metal or wood using a hammer and tools.
Many people often get it confused with engraving or lasering, but those are very different processes which are usually done using mechanical and electrical equipment. Hand-stamping is the process of displacing metal using force, Engraving removes it using a tiny chisel-like tool and Lasering uses a laser beam to burn the surface of the metal.
There is an art to hand-stamping, primarily learning your alphabet in a very tangible sense... Each letter (depending on size and surface area) requires a minor adjustment in the amount or force required to mark the metal. A lowercase i for example would need a much lighter hit from the hammer than an uppercase M. Then again, a think-lined serif font would need less force than a thick-lined italic. A 5mm sized one more force than a 2.5mm one, and so on and so on...
There is also the metal to consider too. Aluminium is a wonderful medium to use for hand-stamping. It is fairly soft in it's base form so can be stamped without the need of a very heavy hammer, but once it's 'work hardened' it is very strong whilst remaining lightweight. This makes it perfect to use for things like Trainer Tags and medical bracelets.
Stainless Steel, Copper, Brass and Sterling Silver are much harder to stamp and require more force from larger mallet style hammers or in the case of silver I usually anneal it (heat it to make it slightly softer) prior to working on it with the design stamps or letter punches.
The rest is just practise... and TONS of it. The more you do it the more muscle memory kicks in and you can stamp without having to overthink all of the rules above. Yes, 5 years on and I still make mistakes here and there but a lot of it comes like second nature now.