Open shoulders (or shoulder flexion) are very important for achieving good handstand alignment. Let's look at a few tips for assessing and preparing for shoulder "open-ness".
Assess Your Shoulder Opening
How to Open Your Shoulders
Get into puppy pose, but elevate your elbow tips on a block or two to give yourself more space to drop your chest and open your shoulders.
Set up a block under your head and one under your mid-upper back. You can vary the height of the block under your back depending on how open your upper back is. Lay on the blocks and reach over your head trying to touch your thumbs to the floor. Remember to keep your ribs pulling down towards the front of your pelvis and belly button into your back. This gets hard as your thumbs get closer to the floor!
Sit with a tall spine. Hold a block by its sides and push it to the ceiling. Your ribs might want to lift up and poke forward, but draw your ribs down toward the front of your pelvis and push up thinking about lengthening through your armpits.