Hi Regina!
I understand if you're sceptical to the cheapo Chinese degausser.
If you've got the cash for the Cfixer, then that's perfect. It's a great gadget for the purpose. Isn't it rechargeable and ready for use on your flight spot?
If, on the other hand, you decide to go for the Chinese one.. I would use it like in the video below.
Touch drone (compass location), activate degausser, move slowly away. All 3 axes.
Then proceed to make a proper compass and IMU calibration, away from steel, wires and concrete.