the font locations are:
~/.fonts/→ for your own fonts
/usr/local/share/fonts/→ system-wide, user installed fonts.
/usr/share/fonts/→ system's font locaton.
just copy your TrueType font (.ttf) into one of the dir.
or, you can launch a font viewer, by clicking on the ttf font or in terminal type
sudo gnome-font-viewer fontfile.ttf
then click the install button there.
If you get error (possibly because a earlier version is already installed), then just manually copy the font to the dir as instructed above.
In terminal, type
sudo fc-cache -f -v to clear font cache.
Restart each app to use the new font. Or, relog.Disqus