Essentially, you buy any lower, file the paper work, and then get your lower engraved when you get your stamp back.
Many ways to have the engraving done. Can find a machine shop local that knows what they are doing, or google a place. I happened to find one in my state that has an FFL that I shipped to and had it back in a week.
Basically, the lower will read...
Booty McBoot Face Or Name of the Trust
Camp Swamp Ass, NC
223 Wylde
The caliber is optional, as long as it is visible on the barrel it is not needed.
The company I hyperlinked did mine and it came out perfect.