To breathe, in yoga, you need a comfortable sitting posture.
You can simply sit in Sukhasana, Put as many cushions as needed underneath your bum and knees.
If you are comfortable, you can sit in Siddhasana. Bring one foot between you chin and your tie.
If you are comfortable and super flexible, you can sit in Padmasana, the famous lotus posture!
I this does not feel comfortable, do not worry! You can of course sit on a chair. Try having both feet planted on the ground.