I'm using Debian 9 x64 with the dwm window manager. I don't use Gnome, KDE, Xfce, or any other desktop environment; it's just the window manager.
I configured my keyboard layout to use the Generic 105-key international keyboard. I've tried AltGr (the right Alt key) plus various alt codes, to no avail. I've also tried AltGr and then " u or u ", with similarly disappointing results.
Ideally I would be able to type these characters anywhere, including text editors (all of mine support Unicode), web browsers, etc. I don't use LibreOffice, so some of the LibreOffice-specific solutions I find aren't enough.