Sitting is not a casual affair in yoga. Perhaps sitting never is.
It's about your back, your spine. Your breath. Your pelvis. Among other things...
I'm talking here about sitting on the floor, with your legs crossed. Although, of course, there are a great many ways to sit...
I'm acknowledging that I am assuming you can get down to the floor to sit. And that's quite an assumption. And, sitting on the floor may not be for everyone. I certainly wouldn't have been doing it when my spine was recovering from acute, multiple level, disc herniation. Or in acute pain. Please don't.
That said. Let's sit.
Take enough support under your buttocks so that your knees are no higher than your hips. You want the space in your hip pit and lower abdomen-left and right, to feel open, spacious, not compressed. Folded firm yoga blankets help. You want to retain the natural curve, the natural lordosis of your lumbar spine. Have a yoga block each side of you, on the floor.... Ready for later.
Notice which leg you've chosen to have crossed in front. Why? Is it habit? If you have a spinal asymmetry, is the leg that's crossed in front, the same side as your lumbar concavity? What happens if you switch the cross of your legs, so that your front leg is the side of your lumbar convexity?
Where does the weight fall through your ischial tuberositities (your sitting bones)? Is it weighted more on one side more than the other? What happens if you (using your hand) move the heavier sit bone back a bit, and the lighter sit bone out to the side a bit? Hmmmm...
Find the settling of your sitting bones way down, onto the earth's deep interior. Sit right ON the sitting bones; not rocked forward or back on them. Watch your spine rise in gravity's Ground Reaction Force. Light.
Place your palms on your blocks, have them roughly beside your hips.
Now, here's the thing. As you gently press your palms to the blocks, keep your elbows soft (don't lock them), keep your shoulders free of your ears (don't hunch or raise them). And PLEASE: don't thrust your chest forward and up. Just don't. This 'lift the chest' stuff (you know, the old 'open the heart story), often results in a hardening, everywhere. Your back, your throat, your nervous system. Plus, it hardens the area behind your heart ( you know, where the blood actually returns to the heart from oxygenating the heart muscle itself: your coronary sinus, in back of the heart.) Good to keep that open! What happens, if instead, you allowed the buoyancy of your airy lungs (that'd be the front, sides and back, top and bottom of your lungs), to lift and open your rib-basket? What happens if you allow precious expansive s-p-a-c-e to penetrate the muscle fibres and chambers of your beloved heart? All four chambers of your heart, front, back, both atria ( the top two chambers) and both ventricles (the bottom two chambers). Spacious. Open. Naturally buoyant and 'lifted'. From the inside out.
What happened in your spine when you did this?
What happened in your abdomen?
Feel the movement of your breath.
Skin soft; your face, lips, your throat - your thyroid as wide and open as your lungs. Iris diaphragm of your eyes wide, soft, open, your spacious eyes unplugged from behind. Brain unravelled...
NB Many folk in yoga advocate sitting with your back against a wall. I don't. Quite simply, the shape of my spine, buttocks and pelvis are such that my back simply doesn't touch the wall; I'd have to lean back quite some way to touch the wall with the surface of my back. I may, however, use a block, or a wedge, at scapula (shoulder blade) level between myself and the wall, but that's another story...!