Pretty simple if you plan to comply. Locate an FFL that offers serialization services, take in your frame/receiver and it's just a simple gunsmithing service. No different than taking a firearm in for some trigger work or a cleaning. No ID or BGC is required.How does one even go about it?
I haven't looked into it enough to know of places in/near PDX, Salem or other... for that matter (I'm sure there are)... but Highline in Newberg and ADR Tactical in Springville are a couple I'm aware of that do it. Both charge the same. $50/ea.