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.

OpenType

cp font.otf /usr/share/fonts/opentype/

TrueType

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.