For Mac: Open up Font Book. If you have multiple font files, your font folder might be zipped. Double-click the folder to unzip it. Then, select all of the font files and drag and drop them into Font Book.
For PC: If you have multiple font files, your font folder might be zipped. Double-click the folder to unzip it. Right click on one of the font files so that the menu opens up. Click Install.
Fonts are stored in the C:\Windows\Fonts folder. Another way to add fonts is by dragging them directly into this folder.