New Monkey C Developer: How to background process

Hi, so I'm working on a watch face that I need to get sensor data for (specifically Magnetometer data). From what I've been able to find, the only way to do this is to retrieve the data from a background service.

My issue is, I can't find (or maybe I'm not understanding) a straightforward explanation on how to call/create a background process and get sensor data as a return value.

Can anyone help me out here?

Thank you!