Upward facing dog tutorial with modifications.
Keep your shoulders stacked over the wrists, and as you inhale expand your chest forward and up. As you exhale, plug the navel in towards your spine. Keep your thighs contracted and elongated, as you maintain suspension of the hips off the floor. Keep your collarbones wide, as we are trying to feel this pose in the upper back as a front body opener rather than any pain or discomfort in our lower back.
To modify, try it with blocks under your hands. If still uncomfortable on your lower back, modify by doing baby cobra, focusing on sliding the chest forward as you keep your navel to the mat and elongate the rest of your spine.
It is a posture often done quickly in classic sun salutations, so it's important to pause if you need to during a flow, to make sure you're breathing (typically INHALING) as you enter this posture with wide collarbones. Pause and find your alignment here, and it will help you maintain good form next time you flow/transition in and out this pose.