Thanks, I have updated the top post. The original post is now in a spoiler at the bottom of the post.
I am not sure if we should provide installation instructions, or if those kind of things are better kept in the “Using LineageOS” thread.
Feel free to edit, it’s a wiki! 
Edit: Regarding the modem firmware, I’m happy to update my flashable zip whenever a new modem firmware version comes out. However, Fairphone could support the custom ROM development by providing an official flashable zip with the latest modem firmware (without /system and /boot) at code.fairphone.com.