The source files are under the sources subdirectory.
The .sfd files are FontForge source font format and should be edited with FontForge.
The .fea files are OpenType feature files and should be edited by a plain text editor.
To build the fonts locally, you will need to setup Fontship. Several methods are available for installation including a simple one-liner with no installation using Docker. One Fontship is setup, regenerate the fonts at any time using:
fontship make
A remote CI runner will also automatically run fontship for all PRs on this repository, so in some cases you may not need to install it at all.
You can even download and review the fonts it builds after each push.
Hovever this is cumbersome for actual font development and we recomend checking your work with local feedback.
Note that FontForge adds unnecessary clutter to its source files on each save that must be removed before committing.
After modifying the .sfd files, and before committing the changes, you can automatically clean them up with:
fontship make normalize