Easy how to : Open Applications > Accessories > Terminal

sudo cp -rf /homedir/user/folder-source-fonts /usr/share/fonts/truetype/folder-dest-fonts

sudo fc-cache -f -v

At command fc-cache -f -v, you will see your font directory and amount of your fonts, that will be installed.

For previewing the fonts, you can use gnome-specimen apps.

sudo apt-get install gnome-specimen