Normally I do all of my image manipulation on Windows with Photoshop as I am not a big fan of GIMP, so I’ve never had to install a new font before. Not immediately obvious but you can do it like so.
1rl@bloodandguts:~$ sudo mkdir /usr/share/fonts/truetype/robl/ 2rl@bloodandguts:~$ sudo cp /home/rl/Desktop/HorsePuke.ttf /usr/share/fonts/truetype/robl/ 3rl@bloodandguts:~$ sudo fc-cache -fv 4/usr/share/fonts: caching, new cache contents: 0 fonts, 3 dirs 5/usr/share/fonts/X11: caching, new cache contents: 0 fonts, 6 dirs 6....... 7/usr/share/fonts/truetype/robl: caching, new cache contents: 1 fonts, 0 dirs 8....... 9fc-cache: succeeded