You have to list the date you completed it, a serial number you made up, your name and address, your birth day (not date), and a statement you can legally possess a firearm on organic unbleached paper. Next you must thoroughly wet it with hot bacon grease, let it cool, then entrust it to your best friend (your dog) for safe keeping. You can borrow a best friend if you don't have one currently.
Seriously, as Jim said above, there are no laws requiring registering a homemade gun in Colorado or at the federal level right now. You don't have to put any markings on them either. Except the way I read it the cartridge chambering must be marked on the gun. That can be on the barrel, it doesn't have to be on the receiver.
Have a Good Day.