Choose the installation scope
For one user, GNOME documents the ~/.local/share/fonts/ directory. Put the extracted font resources there. For all users, its documented location is /usr/local/share/fonts/; writing there needs appropriate system permissions.
Choose the smaller scope that fits the actual need. A font for your own document workflow does not automatically need a machine-wide installation.
Refresh discovery when needed
The documented cache command for a personal installation is fc-cache ~/.local/share/fonts. For the shared location, the corresponding example is fc-cache /usr/local/share/fonts/. Restart an application that was already open if the new family does not appear.
These instructions concern applications using fontconfig. An application with its own font catalog may use a different workflow.
Choose a font resource deliberately
Read font file types before selecting files. An archive needs extracting; an image of lettering is not an installable font. Keep the included source and license with the project.
Compare a heading, punctuation and the language you actually need. Installation alone does not establish that a family contains every required character.
Separate local installation from cloud editors
Installing a font locally does not prove that a browser-based editor will add it to its own catalog. Follow the application’s supported font workflow.
For another operating system, use the Windows installation guide or Mac Font Book guide.
Your questions, answered.
Must I restart the whole Linux session?
GNOME notes that running applications may need restarting, while a user-session restart is not required for this documented workflow.
Does this add a font to every web app?
No. A web editor can use its own hosted catalog or upload workflow rather than the machine’s fontconfig resources.
Sources & background
Sources supporting this page’s explanations. Practical examples are our editorial suggestions.