There's a feel to it that's hard to explain, but it's similar to when you first start riding a bike and finally something "clicks" so that you can stay upright while pedaling.
After it "clicks" you get a distinct feel for when the bike is actually going to fall over and when its just leaning a little but still "in the pocket"
Try this:
If you're familiar with yoga, stand in Mountain pose, arms stretched overhead like you're reaching for the sky. Glutes and core squeezing and strong but not clenching.
Without releasing your core or glues, bring your arms down by your sides, elbows bent, so you look like a Shaolin Monk about to unleash some Kung Fu.
That's about how you'll want to hold your body when you're in your spin.
Next up, while skating, begin by starting to transition from forward to backward, only instead of using your momentum to begin moving backward, you continue to bring it around and UP.
For the 2 foot spin, you'll want to keep your feet close together, and like someone else said, it's easier to do on the "heel" of one foot and the ball of your other foot.
Think of it like Yin & Yang if that helps.
Go slow and figure out the exact foot position for your body and your skate setup, it's going be slightly different for everyone because we're all a little different. You may have to squeeze a little to keep your feet in the right spot once you start spinning, and you def will have to once you get some speed going.
*Remember the Kung Fu Monk* for how to hold your core and body while spinning!
For the 1 foot spin, you'll need to do something called hip-stacking
To get an idea on how to do this, stand with your feet together and arms loosely at your sides. Now without bending your knee or ankle, lift a foot off the ground using only your hips, but also while keeping your leg directly underneath you (You're not sticking a leg in front or behind you like walking, but literally lifting a hip upward towards your shoulder.)
After that, you're looking for the balance point on the ball of 1 foot.
It may help to start by using your toestop as the 3rd leg of a tri-pod (toe-stop spins are a thing! You just won't get very many revolutions and it doesn't look or feel as fun!)
The only other tip I can give on those is to buy one of these things:
Spin Trainer
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That and just keep at it. Practicing off skates on a wood/tile floor with some thick socks on is surprisingly helpful too!