1. Download the fonts and extract the zip/tar.gz

e.g. tar -xzvf FooFonts.tar.gz

2. Copy them to the fonts directory (/usr/share/fonts)

cp -r FooFonts /usr/share/fonts

3. cd /usr/share/fonts/FooFonts

4. Create fonts.scale and fonts.dir files

mkfontscale

mkfontdir

fc-cache

5. restart gdm (xserver)

/etc/init.d/gdm restart