You must maintain a "bound volume" of acquisitions and dispositions.
It CAN be kept on computer. You CAN buy a printed form from Brownells. Me, I went by Dollar General, got a small stitched journal type book, used a ruler, set up lines and columns- in two sections.
Front of the book has "Acquisitions"- guns I acquired- bought.
Date, type, action, make, model, caliber, serial number, from who, where.
Typical entry may be:
1/21/15 Rifle, bolt action Remlin 12-A, 22LR, No serial number, Joe Doaks Guns, 33 Mockingbird Lane, Sterling, West Carolina.
1/22/15 Shotgun, double, A H Fox, Sterlingworth, 12 g, SN 1234, Booby Broke, 12 Oak Lne, Hampton VA
If I sell or give away a gun- same thing, in the Disposition section.
Modern guns you bought outside of your license do not go in your bound book. ATF wants you to list any C&R guns you get, even if you did not use your license (face-to-face buy from a non-dealer) but I do not see that in the regs. But I do it.
When you renew your license, you will be asked how many you acquired, how many you disposed of- easy to pull out of your book.
If you ever destroy a gun (bought a non-working Marlin 81 for parts, did not need receiver) destroy the part by cutting it with a torch into 3 pieces, and in disposition, list it- but for name of recipient put DESTROYED BY TORCH CUTTING.
I also keep a large envelope taped inside back cover- if I got a gun from you, and you sent me a copy of your license, tuck it in there.