Most breathing exercises begin by telling you to sit in a comfortable position with a straight back. When you slouch, shoulders rounded and head forward, your ribcage and sternum press against the diaphragm, tightening the muscles around the chest, preventing the diaphragm from working efficiently. Needless to say, slouching is bad for you overall, especially for your neck and back.
Some breathing exercises may require you to place your feet on the ground or to assume a specific pose, such as lotus. But more often than not, sitting comfortably (as long as your back is straight) is what’s important. This way, your muscles can work effectively, and you can concentrate on your breathing without constantly fidgeting. Never underestimate the importance of getting comfortable in a breathing session and get into a position that you can keep for 10-20 minutes, depending on the session you’re practising. If you’re sitting on the floor, make sure to have a cushion under you.
You could also lie down in a breathing session, especially if diaphragmatic breathing is new to you, which is much easier to practice when lying down. Keep in mind, however, that you risk relaxing too much and falling asleep in a lying position. You are much more likely to be awake and focused in a seated position.