Hi KJ20 and welcome to the Forums!
You can start and stop recording with 24/7 Video Recording. From your Xfinity Home Mobile App, tap on the Automation button, tap to add a new rule, and then choose the "Put 24/7 Video Recording on a schedule" to get started.
Videos are automatically deleted after 7 or 10 days, depending on the area you live in and service you subscribe to. If you wish to save a recording for longer, you can download it to your device or computer from within the App or at http://home.xfinity.com
I hope that helps!