This is helpful and shows how to breathe well if you get ill, and is recommended by this doctor to do even before being ill.  It’s only 3:42 minutes, and the doctor is clear and shows how to do the breathing at about halfway through the video. He does note that doing it made him a bit dizzy so be aware of that. I hope it helps somebody.  Be well all.

https://www.reddit.com/r/videos/comments/fw1msc/doctor_at_queens_hospital_in_the_uk_advises_on/

best wishes
Donnalee