Go here and download the NFA Handbook-
http://www.atf.gov/publications/firearms/nfa-handbook/

It has a plain language explanation of what's required that also references to the regulations (which are in the back).

Some people make their marking as inconspicuous as possible and other put it in the open. It depends on what you want.

I always try to get information from the ATF handbook first. If I don't understand it then ask on the internet.