Now it's over to some random gist (https://gist.github.com/szepeviktor/2c6a19cb91c4bb561369707f22bcf413 "Signal on Windows without a smartphone" although I'm not on Windows) because this is apparently The Authoritative Guide to doing this unofficial thing.
The guide asks you to upload a QR code to a completely unaffiliated barcode-decoding site at one point so that you can decode it. Is this safe? Who knows! Probably not!
(I used a local program. Probably most people don't do this.)
Now Signal Desktop is paired or whatever...