This is just like every other package recipe for Arch. We provide a
PKGBUILD and then
pacman does the rest.
makepkg -si. This synchronises the source, compiles it, and
installs the resulting files using
emacs-git-188.8.131.52666-1-x86_64.pkg.tar.zst. Get its path and
pacman -U /path/to/package.pkg.tar.zst.
makepkg -si. Sometimes you
will get a build failure due to some massive change upstream. In
that case, run
-C flag also does a
clean build and thus takes longer.
emacs-git from the AUR?
Use that. I have not tested this thoroughly and do not recommend you try it. I simply wrote it as a way to learn how Emacs is built and ultimately particularise my configuration of it.