It can be done with jigs and a hand drill, but doing so is a clusterf*ck. If you have a mill, it's still not really worthwhile.
There are two reasons I believe folks do it: 1) to say they did, and 2) because they have some tinfoil hat condition where they want a "ghost gun." In most states, your gun is MORE of a ghost if you buy a used rifle through private sale than it would be if you purchased an 80% lower. As an example - I have LOTS of firearms I've legally purchased with cash, no paperwork anywhere. Alternatively, every time I've ordered 80% lowers, the selling company has a sales invoice record, and my credit card statement reflects the purchase... Which one is a ghost?
I built a pair of 80%'s for my wife and I, serial numbers (not required, except the fact I SBR'd them) are our initials and wedding date, and custom laser engraving done on the magwells. It was something special to say I did it, and the only reason I bought 80%'s was to have absolute control over the engraving locations and designs. I paid WAY more in the long run than I would have to have simply bought a couple lowers cash, or rifles from an individual.