I needed to install a font to create an image of text for a friend, so this is the quick and dirty way I did it.
I dropped the font into into my Downloaded folder, then in the Terminal,
username@yourcomputer:~$ cd Downloaded
username@yourcomputer:~/Downloaded$ sudo su
Password:
root@yourcomputer:/home/username/Downloaded# mv MISTRAL.TTF ~/.fonts

As I say it’s quick and dirty but it worked. [...]