Let's say your downloaded fonts are in
fonts directory in your Downloads folder, Do these to install them
If you want the fonts to be usable by all users
/usr/share/fonts with command line
sudo cp -r ~/Downloads/fonts /usr/share/fonts/truetype/
Alternatively, Type this command in terminal to open graphical file manager if you prefer that way
sudo -i nautilus /usr/share/fonts/truetype
and copy the fonts from the downloaded directory to the newly opened folder in nautilus
If you don't bother for all users
In this case, the recommended way is to copy the fonts in
~/.local/share/fonts directory (It is not recommended to create
.font directory directly under home anymore). When you put the fonts in that specified directory, Ubuntu will automatically start caching process. After 5-6 seconds, the fonts should appear in listing. Close any open application before you want to change the font for it.
Fixing the font files' permission bit
Font files should have the permission of
644 and the containing folder should have
755. So if you see don't see the fonts in listing, please check the permissions of those files and folders.