Once you get comfortable doing Pranayama, you will be sitting most of the time. Whatever seated position you choose, have your knees level with or lower than your hips. Keep your pelvis upright, not tipping back so that you are struggling to stay up, and not tilting forward so that your abdomen is falling out of the pelvic bowl.
I am pretty tightly woven, so I tend to sit on a high support. Using a belt in this way gives you a very steady seat. The belt is just below your waist at the back, across the top of your pelvis. Position the buckle to be easy to adjust, with the strap end coming toward you. And oh, please, don’t let that hard buckle be right on your knee!
Virasana is a very stable pose.
Sit between your feet, and keep them pointing straight back.
Sitting against the wall is a good transition when you are learning. The wall gives you helpful feedback – where IS your back? When you first sit down at the wall, put your hands beside you, lift your buttocks and lean forward to push them back a bit close to the wall.
Cranky knees? Practice sitting on a chair. Keep your knees over your ankles and your pelvis upright.
I learned this way of sitting from my teacher Kim, who learned it from Geeta Iyengar. Heels on the outside of the chair and hands pulling outwards on the back help to keep your chest and hips open. Why do we want open chest and hips? Better breathing, of course.