Use fontconfig !

All applications I used on Wayland relied on Fontconfig, which means
that you can just put font files in the following places, and expect it
to work:

/usr/share/fonts/
~/.local/share/fonts
~/.fonts

Everything you know about fontconfig still applies on Wayland, so there
is not much to say about it. Simply lookup fontconfig examples on the
web and be done with it !

Something worth mentionning: You MUST have at least one font available
on the system. My compositor just wouldn't start because it couldn't
find a single font to match "monospace", and as I was starting it from
the VT directly, I couldn't see the log, and started pulling my hairs
off trying to understand why it would crash. I really didn't suspect
fontconfig…