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 Windows: 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.
For Chrome Book: Install new fonts via the app you want to use them in. For example, if you want to use the purchased fonts in Canva, open Canva and follow these instructions: https://www.canva.com/help/upload-fonts. If you want to use another design app, try googling “install custom fonts on [insert the app name you want to use the fonts in]” to get more specific instructions on how to install them in your specific app.