These instructions documented here: 
https://docs.google.com/document/d/1hjEWX06Z53OpV2gh1Wdm1S14dORZhB3whmzI2KUoXw0/edit?usp=sharing

In my home office setup, i have my laptop, and my mobile phone, with Zoom app installed on each.  I join the zoom session from laptop and turn on my microphone and loudspeaker.  I then join the same zoom session from my phone and just turn on its camera (I do not use the sound on my phone, i only turn on the camera) .  I use a shoebox or similar to orient my phone to point down at a piece of paper where I can write equations.   The host can use Zoom's spotlight control to force everyone to see the camera shot from the phone at all times.  How to spotlight: https://support.zoom.us/hc/en-us/articles/201362653-Spotlight-Video