Actually, my wife used an app and a device to help her learn to breathe better.
The app was free and somewhat close to what the device encouraged.
Look up “The Breathing App” and iBreathe (by Lukasz Gryl). Both are free on the iPhone, with no ads.
Set The Breathing App at 5:7 and when you adjust to that, make your custom one in iBreathe that is longer.
Check breathing apps
=10 for later