Whether or not you are new to yoga, here are some things to keep in mind about the breath:
Both the inhalations and exhalations are through the nose. Unless your nose is stuffed up, close your mouth and breathe through the nose. If you’ve lost your breath, back off of the posture a bit until you catch your breath. No panting or gasping, please. If you are pushing too hard in the physical posture, your breath will let you know.
About three seconds in, three seconds out. We are constantly working to equalize the lengths of the inhalations and exhalations, especially watching that the inhalations don’t become short. The inhalations, especially in transitions, are sometimes rushed. Be mindful.
The breath is a soft, soothing breath, a medium breath. Not as soft as natural breathing as it does have a gentle sound. However it is never a forced breath - never forced or harsh. You can hear your own breathing and probably your teacher can if he/she is right next to you. Darth Vader is not practicing yoga, so be careful that your breathing isn’t harsh and distracting to others (especially the exhalations). No grunting and/or groaning.
Fill your lungs, empty your lungs. Take complete inhalations and complete exhalations. Deep diaphramatic breathing, but not a belly breath. The lower abdomen stays drawn in.
Don’t hold your breath. Work to keep the rhythm with your breath as you set up a posture or if you fall out of a posture. If you are new to yoga and the breathing is a challenge, be patient. It takes time, just don’t hold your breath.
More on the benefits of nasal breathing here:
Here’s a video tutorial by Jason on breathing: