What are the requirements for engraving NFA markings in firearms?

The ATF mandates a very clear set of specifications for NFA engravings

As outlined in the Code of Federal Regulations markings must be a minimum of 0.003" in depth and the serial number shall be in letters no smaller than 1/16th of an inch in height.

Additionally the firearm must be marked with the names of the manufacturer (your name if registering as an individual or the name of the trust, corporation, or LLC registering it) as well as the city and state in which the manufacturer resides.