A file prompt window should now be visible, showing your downloads folder. Right-click on the Alohomora extension file, which will be a greyed-out .zip file named "alohomora-1.x.zip" or something similar. Click "Extract All" in the menu that appears. An unzipped folder should now appear. Double-click the unzipped folder, then click once on the folder inside. Finally, click the "Open" button, located on the bottom right of the file prompt window. This will install the extension.

Once your extension is unpacked and installed, click on "Keyboard shortcuts". To set your shortcuts, click on the pencil and type in the shortcut on your keyboard. You can also set them as your own shortcuts to your preferred shortcuts. I suggest leaving the Ctrl+U shortcut unfilled, as this can cause issues when using apps such as Google Docs, and is not necessary except for developers.