Run ./build-image.sh. This will download Buildroot and build a Beepberry SD card image at buildroot/output/images/sdcard.img.
For subsequent builds you can just run make -j $(nproc) from the buildroot directory.
For the keyboard to work, you'll need to build and flash the firmware from this PR for now: sqfmi/i2c_puppet#1
Create a file at root_overlay/var/lib/iwd/<SSID>.psk with these contents:
[Security]
Passphrase=<passphrase>
Next time you build an SD card image, this network will be configured to auto-connect at boot.
SSH is enabled by default, you can configure WiFi as above to connect remotely to debug.
The username is beepberry and the password is beepbeep. You should change this from the default using passwd as soon as possible for security reasons.