A notification will appear at the status bar when the app is downloading.

Once it has finished, deploy the status bar to show the download notification.

Press on the notification to begin the application install.

If the system does not allow to install the application (with the reason that it comes from an unknown source, as it has not been downloaded from Google Play) access your device settings to enable the "Unknown sources" option.

This option can be located inside the "Security" menu.