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. I opened Open Office and there it was, fully functional.

I should have also added (and later did) fc-cache -f -v ~/.fonts

username@yourcomputer:~$ fc-cache -f -v ~/.fonts

That produces a fonts.cache-1 file in the .fonts folder. Not sure why.

For the proper way to install in Ubuntu, soee the instructions and comments at Penguinfonts.com