Easy-peasy-lemon-squeezy. It's a two step process:
1) Copy the font files to the correct location
There are two types of font files you can use: OpenType and TrueType. OpenType fonts have a .otf file extension. TrueType fonts have a .ttf extension. Make sure you put the files in the correct location.
cp font.otf /usr/share/fonts/opentype/
cp font.ttf /usr/share/fonts/truetype/
2) Run this command to refresh the font cache:
sudo fc-cache -f -v
You should now be able to see your new fonts on the system, but you might need to close your application and relaunch it for the change to be detected.