I have switched to using fontmake to build the fonts, this should make the process somewhat more transparent.
In any case you need to start from a Glyphs source file with a large enough design range and character set to accommodate all the weights and characters that macOS uses. Some of the additional glyphs that I found are required are included in additions.glyphs. These need to be copied to your Glyphs file, and matched to the design and dimensions of your font.
The Glyphs source then must be modified to contain the correct font instances in the correct order. I used to do this by running text_roman.py inside Glyphs, but I didn’t check if the results are still OK. I fiddled a lot manually for the latest release.
If you have the modified Glyphs file, you must enter its path at the beginning of build.sh, and then execute build.sh. If you have fontmake and all dependencies installed, the rest of the build process should go smoothly and you will find the final font in ttf/FSText.ttf.
If you are in doubt if you are on the right track, compare your generated .designspace file with the one in the repository. Note that most of the temporary files are hidden by default because the font names start with a dot. Use the shortcut cmd-shift-. to view all files in Finder.