There are two packaging options for you to get Fingerboard. You can use a package that builds it from source or you can just install the compiled binaries. If you don't know what this means, you want the compiled binaries. To get the PKGBUILD, run
wget https://raw.githubusercontent.com/grelltrier/keyboard/master/packaging/bin/PKGBUILD
or
wget https://raw.githubusercontent.com/grelltrier/keyboard/master/packaging/src/PKGBUILD
Once you have the PKGBUILD, run
makepkg
pacman -U [NAME_OF_PACKAGE]
to install it on your device.