How to add fonts on Libre Office Writer?

Fonts that you download from the internet (ttf and otf files) can be installed and used in LibreOffice Writer and other programs by placing them in the /home/<your username>/.local/share/fonts directory. If it does not exist you will need to create it. Though there's no need to muck around in a terminal or copy files around. If you double click on a font file in the file manager, Font Viewer will automatically open. You can then install the font with the press of a button:

There are also a good number of fonts available directly in the Ubuntu repositories. These can be installed just like any other package. You might also be interested in TypeCatcher which lets you view and install Google Webfonts for local use.

LibreOffice will read all installed fonts in /usr/share/fonts/, which is where fonts packages will be installed by the Software Center, and are accessible to all users (except if it is a LaTeX font package, but that's another story).

Additionally, if you copy/download individual fonts, you can put them in your ~/.fonts/ folder (or double click on the font file and click the button "Install Font"), which will make them available only to the current user. You can use these fonts in LibreOffice too.

If ~/.fonts/ does not exist, you can create it from the file explorer or from command line:

mkdir ~/.fonts