Standing head to knee is one of the hardest postures in most yoga systems, which is saying something, because half of yoga is just standing on one leg pretending you meant to do that. The other standing balance poses are easier by comparison. If you're feeling aggressive and fearless, and you think you can do it, go for it. Don't hesitate. Your body will let you know if it disagrees, usually loudly and without warning.
But if you'd rather learn it methodically instead of finding out the hard way, here's what I can tell you as someone who can do all the phases of standing head to knee, including the one you won't see in class, letting go of the foot entirely, which is less a pose and more a personal vendetta against gravity.
The first step is figuring out where your points of power actually are. Press the toe firmly into the ground. Tighten the calf muscles, front and back. Extend that contraction up through the thigh, into the glute, into the oblique, into the lower abs. Then pull in the pelvic floor. It takes years to learn how to move the pelvic floor on purpose, mostly because nobody hands you an instruction manual for it. It's the space between the anus and the genitals, kind of like the taint but deeper in, very romantic, I know. Look at a body diagram and you'll see exactly what it is. It's a root lock, the root of all your movement, coordinating the lower half and the upper half. When you can pull in the pelvic floor and the navel together, it becomes nearly impossible to hurt your back doing an intense forward fold. So lock all of that in before you go diving for your shin.
If you find yourself wobbling and falling out too easily, slow down. If you wobble out enough times in a row, your mind starts expecting it, and congratulations, you've accidentally trained yourself to fall. Instead, stop, and be satisfied with the best version of the pose you can hold without tipping over like a poorly parked bicycle. The rest will come on its own.
Remember, the most important thing in this posture, and in the entire yoga practice, is to breathe full, deep breaths through the nose. I've written plenty about nasal breathing on the Goodsugar website, because apparently people need convincing that their nose is for more than holding up glasses.
I'm glad to hear you're a hot yoga enthusiast. In my view, it's one of the best mental and physical therapies you can give yourself, assuming you survive the room temperature.