Just go out and take some lessons. Any kind - English/Western. Get referrals from someone you can really trust - ask around. Tell the instructor you are a beginner. Just because you can ride a horse without falling off doesn't mean you aren't a beginner. A good instructor will make sure your ground work foundation is solid. Basics in horse care and barn maintenance - and even mounting and dismounting need to become rote. Then the lessons will start with foundation stuff - like hand and leg positions which is what we first see in your riding pictures.

I didn't ride for many years but when I got on for my first lesson in 15 years the instructor was pleasantly surprised how my body snapped into the correct positions. When I first went to her for lessons, btw, I told her my riding level was "beginner to novice" even though I used to show quite a bit and can produce a blue ribbon in jumping I got when I was 17.

Not my circus.  Not my monkeys.
      -Polish proverb