Setup and Key Actions of Downward Facing Dog
Begin on all fours with your knees slightly behind your hips. Line up your wrist creases parallel with the front edge of your mat, spread your fingers evenly apart, and resist your thumbs toward your fingers, and your fingers toward your thumbs (like you’re "clawing" the floor). Press the mounds below your thumbs and index fingers firmly into the ground. Tuck your toes under and press your hips up and back into downward dog.
Bend your knees to start, and make sure they point straight ahead (err on the side of moving the knees out toward your pinky toes while continuing to root through the balls of your big toes). Keep your finger pads and the mounds below your fingers rooted and maintain the clawing action in your hands (resist rocking back onto your wrists and lifting your fingertips!). Externally rotate your upper arms to broaden your upper shoulder blades and release tension in your neck. Align the back of your skull with the back of your pelvis so you're not looking up, but not dropping your chin either. To straighten your legs, press the tops of your thighs back, but if your low back rounds, keep your knees bent.
Modifications of Downward Facing Dog
For a gentler (less weight-bearing) variation, practice standing with your hands against a wall or on a chair.