You have to put the .ttf file in your home, particularly in the .fonts folder (and create the folder if it's not there).
Asuming you have your font (
myfont.ttf) in your Downloads folder (
/home/**yourname**/Downloads -which is the same thing written in a different way-), the commands you should run are:
mkdir -p ~/.fonts
cp ~/Downloads/myfont.ttf ~/.fonts/
Modern versions of Ubuntu (from 12.04 and on) doesn't need you to run a font-cache update. The system will pickup your new font in a pinch. :)