When you install FontBase for the first time, you will see that Google Fonts are already loaded and you can use them even without an internet connection. Please keep in mind that usually there is a delay between when Google updates their fonts available at their main page and when they become available for direct use via Google API, so there may be some minor inconsistencies between the pre-loaded fonts and the ones currently available online. This, however, can be easily remedied by downloading the desired Google fonts and adding them into FontBase as you would any other font.
To add fonts into FontBase, open it and your file manager of choice, then drag the desired fonts from the file manager over the FontBase window and drop them. The files will be added right away (or it might take a few seconds if you choose to drop, let's say, 10 000 fonts in one go) and will be copied into the folder you have currently selected in FontBase's sidebar on the left, or directly into the Root folder if no folder is selected at that time.
You can also add entire font folders by following the instructions above, but dragging and dropping a folder or folders instead of individual font files.
Another way of adding fonts is to open FontBase and click the "+Add" button in the bottom left corner. You will see a dropdown list. Select either Font or Folder, then pick the desired font files/folders and they will be added into FontBase just as they would be when dragging them from a file manager and dropping them over the FontBase window.
Watched folders are a special type of folder in FontBase. To create one, click the "+Add" button in the bottom left corner and select "Watched" in the dropdown. Select a folder on your computer when prompted. You can notice that a Watched folder has a slightly different icon on the left of its title compared to ordinary folders.
Watched folders are links to the folders on your computer. If you add a Watched folder in FontBase, only a link is created in the Root folder, not a full copy as is the case with ordinary folders. Because of this, whenever you make any changes to the folder you've added as Watched, these changes will be reflected in FontBase as well, e.g. adding/removing fonts, renaming or deleting the folder or its subfolders etc.