Shoulders, Abdominals, Spine
Lie on your belly. Stack your forearms on top of each other and rest your forehead on your forearms. Separate your legs a comfortable distance apart, and turn your toes out. Draw your arms in enough that your upper ribs and chest are off the floor (lower ribs remain on the floor). Relax your jaw and the muscles of your face. Notice the places in your body that move as you breathe.
If stacking your forearms is uncomfortable, you can stack your palms and rest your forehead on the back of your hands instead, but make sure that you walk your elbows back far enough to elevate your chest and upper rib cage away from the floor. You can also prop your chest with a blanket, rolled like a tube.