Sit on a firm chair of standard height, like a dining chair without arms. Sit in the middle away from the back. Have your knees wider than your hips. Put your feet flat on the floor, barefoot or in socks, with your shins vertical.
Let your elbows hang straight down and rest your palms on your thighs. Pull your shoulder blades back and down then let them just hang.
Rock your pelvis front and back and feel the curve of your back move. Find the tilt of your pelvis that feels good for your back. And gently firm your abdomen.
Look straight ahead in a way that helps you feel your neck straight.
And just sit there for a while.