to expand on it
just like ANY FFL you keep a bound book
except you legally only have to record those guns covered by you license (I.E. C&R's)
and then the rules, when you have the C&R you have to record your acquisition and dispositions of them. So if you have a C&R from PRIOR to your license, you have two methods generally used, 1, when you get it, log ALL covered guns in (from yourself) so you don't have to later if you get rid of them.
2, when you sell, you log the gun in from yourself, and OUT to the person you are selling to.