By "tape your mouth", James Nestor suggests taping the mouth shut while sleeping, to condition the nose (extra benefit of not drying out the gums--my dentist approves). It took months of applying a thin strip of tape vertically across my lips (surgical tape, easy to pull off if needed, but it never came to that), and now I habitually breathe through my nose at night instead of my mouth, and can nose-breathe all day, too, where before I'd need some mouth-breaths in a panic.
The switching-nostils breathing is just one way to practice (there's a list at the back of the book). If anything, best practice for oxygenation and venting C02 seems to be inhaling over 5-6 seconds and exhaling for the same or slightly longer. The more-recent book Breath Taking by Michael Stephen backs this up.
Nestor stuff is short. Breathe through nose, tape your mouth, right nostril inhale/left exhale.