I want to be careful to suggest this is not an exercise as such, but a procedure to reduce interference patterns that may be present in your breathing.
You don't need to try to breathe any more than you need to beat your heart. I often say the same about posture, you don't need to do correct posture, you'll be poised, much like you will be breathed.
One of the causes of interference is our stress response, and this procedure will also help to address that.
Interestingly, before FM Alexander developed his work into what we now know as the Alexander Technique (which he simply referred to as The Work), he was known as The Breathing Man. Much of the Alexander Technique was born out of his observations on breathing referring to it as "true primary movement". My colleague Halvard Heggdal wrote a revealing essay on this entitled The Primary Movement. It's probably of more interest for my fellow Alexander teachers, but it's food for thought for all.