To add a watch to the watermelon skin APP, first open the APP and find the "Add Device" option. After clicking this option, an input box will appear, and the CID number of the watch needs to be entered. The CID number is the unique identifier of the watch, which can be found in the settings or instructions of the watch. After entering the CID number, click the Confirm or Add button, and the system will verify and successfully add the watch to the APP.