I took a peek at those Instagram clips you posted. First thing that I noticed, your feet are too perpendicular to the deck.
For generally riding downhill, you want your feet closer to straight than sideways. Maybe 30* turned on the front, back foot will pivot based on turning left or right.
For turning toeside, you'll want your chest to be touching your knee (or thereabout). You will grab rail in front of your front foot with your front hand, then your back hand will reach forward to around where the front toeside wheel is. You start the toeside slide by pushing your legs straight down from this position.
For heelside, it's a bit hard to get in the stance if you're not rolling/turning. You squat over your front foot, back foot is near 90*, and you sort of fold the back knee towards your front knee. Front hand will reach down towards your front heelside wheel.
If you want to practice this in a relatively chill way, parking garages are good for practicing putting a hand down while turning. You shouldn't be going fast enough to worry about sliding out accidentally, but the momentum makes it easier to understand how to turn like this.
There's a good video on the landyachtz channel of people learning the basic hands down slides. I'd recommend watching that, they did a good job of noticing and correcting beginner mistakes.
Another general point: riding down hills is all about your front foot weight. Your weight should be like 70/30, trying to steer with your front foot. Having too much weight on the back is unstable.