It should be documented in the GitHub repositories for the respective libraries.
The gesture to start it is "shake, shake shake, tap, tap, tap".
At present, I believe the mobile companion is only implemented on iOS and Android. Also, you'll need to make sure that you are using the "full" Tealium library (rather than compact), and that you have enabled mobile companion support in the publish settings for the profile you are accessing.
Check out our new Swift integration library for iOS, macOS, tvOS and watchOS: https://github.com/Tealium/tealium-swift with updated documentation https://community.tealiumiq.com/t5/Swift/tkb-p/swift.