Speaking of process...
Let's look at some of the key engagements that will contribute to getting and keeping those heels straight over head. I'm going to split your body up into 3 parts: Arms & Shoulders, Core, and Legs. You'll start your alignment checklist with the part of your body that's attached to the floor, and then work your way up.
DOS AND DON'Ts
1. ARMS & SHOULDERS
DON'T let your arms make a pyramid, i.e. don't let your elbows fling out and/or hands in together. (It's not "bad" if they do, it just means your shoulders are tight. Mine even do this from time to time, but we are working towards proper form. So, try putting a block between your hands to help work towards keeping the hands and forearms in line with the elbow.)
DO have elbows/forearms shoulder-width distance apart and practice keeping forearms and hands straight.
DO spread fingers evenly and grip the ground.
DO isometrically hug elbows/arms in towards your center.
DON'T sink weight into your shoulders.
DO press against the floor with your forearms, and press into your shoulder blades, taking shoulders away from ears.
DO look between your hands.
DON'T jut your ribs out or push your chest out.
DON'T collapse into your low back.
(These are big contributors to the banana shape of the "Not This" picture above.)
DO hollow body.
DO engage core via lifting pelvic floor up and drawing navel in.
DON'T be a banana - don't let your feet fall past your hips, shoulders, or head (unless of course you are working on your holloback/backbends, but these alignment cues are for a straight pincha practice).
DO gently squeeze those glutes.
DO draw your legs from thighs to ankles to hug in together.
DO keep legs stacked over hips, hips over shoulders, shoulders over elbows.
DO hug one knee into chest if the weight keeps wanting to fall behind you. This will keep the abs active which in turn will bring your extended leg straight up and down.
Some good prep poses are:
Uttanasana/Standing Forward Fold
Dolphin Pose (3-legged dolphin & dolphin pushups)
Kiss The Wall Handstand (face to wall)
Disclaimer: Getting completely discombobulated with what's activated and what's not, what's where and who's who is completely normal while learning inversions.
So, I recommend on occasion to record your self practice. Because sometimes you'll think and even feel like "Hell Yeah! I'm owning this pose!" while in it. But, then come time to review and look at your posture you'll be like, "What the? I could've sworn I was all the way up and down!". Don't be discouraged however by how your asana looks, or how pretty it is or isn't. The picture or video is just there to inform you of where you can apply more engagement.
You can always use "paul Wall" until ready to tackle the middle of the floor (but bewaaarre of the banana back). Forearm stands can be tricky, especially when it comes to making sure every part of your body is activated from your fingertips to your toes to successfully pull off the posture. So BE PATIENT. BE PERSISTENT. And ENJOY THE SMALL VICTORIES.
Grow with your flow,