To do the Downward Dog, start in the high plank position with your hands under your shoulders and your feet about hip-width apart.
Without walking your hands back toward your feet, push your butt up toward the ceiling as you press your chest back toward your legs and drive your heels down toward the ground. Keep your arms straight as you press your chest back and try to get your biceps by your ears. Make sure to press through your entire hand and not simply rock toward the outside of your palms. Feel your fingers gripping the ground.
As you hold, focus on getting your back flat and a nice straight line from the heels of your hands up to your tailbone. You want your arms to be in line with your torso (aka your biceps by the ears).
As you press through your hands to get your back flat, focus on also driving your heels down as you lengthen through your hamstrings. Really push your butt back and up as you hold the downward dog position and try to drive your heels down toward the ground.
Breathe and try to relax deeper into the stretch. While you may need to adjust your exact hand positioning, do not walk your hands in closer to your feet just to get your heels down. You want to make sure you are really focused on driving your chest back and extending your spine to improve your thoracic extension and stack your shoulders.
If you are using this as a beginner variation of the Handstand Hold, do not worry as much about keeping your legs straight. Instead you may bend your knees a little in order to help you really extend your spine if you struggle to get full extension with your legs straight.