Fellow Arch user here (btw). It’s exactly the same as building AUR packages. Clone a git repo containing a PKGBUILD, use makepkg to build it, and pacman to install it. The nice thing is you can host a repo of your built packages and install them on other systems really easily. The big downside is that dependency management is not automated, so it will take some time and annoyance to map out what packages you need to build and in what order, if you want a fully source-bootstrapped system.