Hi,
The way I manually add fonts to LL is through terminal.
Lets say I need to install Cardo True Type Font. I will download and extract the files. e.g. /home/myname/Downloads/cardo/Cardoi99.ttf
To avoid any conflict, I have created a new folder under /usr/share/fonts/
for an example we can create folder named "new"
sudo mkdir /usr/share/fonts/new
Then we need to copy the .ttf file in the original location or the new folder created.
sudo cp /home/myname/Downloads/cardo/Cardoi99.ttf /usr/share/fonts/new
Change /home/myname/Downloads/cardo/Cardoi99.ttf to source where you have downloaded and extracted your font files.
We can copy multiple files using wildcard e.g. *.ttf, in case we need to add multiple fonts.
Then we can either reboot our system or can manually rebuild font cache by running following command
fc-cache -f -v
if required you can run the command using sudo.
Post following these steps I was able to get my favorite fonts installed in LL and was able to use in GIMP and LibreOffice.
If there is any easier way you found like using any application/program, let us know.
Bikram