First thing. Is there a known health reason you are doing this? When doing this, you have to take care of your balance and your back when trying this. Luckily, you can cheat a little when practicing for the full stretches. Slightly bent knees count! Just that they don't count for the last 5%. There may be better instructors in this tribe or available via a google search, etc.

Second thing. Yoga helped me do all sorts of things...but it messes with my knees if I do all the kneeling stuff. The thing to know about yoga is you don't have to impress anyone but yourself if you don't want more than that. I studied a book on yoga, and learned all sorts of stuff from it, setting a goal and then went for it. It took me about six months of a half hour before I went to bed. My back had started killing me and I was able to do all sorts of amazing things by practicing yoga on my own. The main thing I learned from yoga was how to breath when stretching...

Third. You can cheat a little at first with slightly bent knees. Here's how I learned to do stretches to the floor. When you are standing straight, put your feet apart so that they are lined up with your shoulders and hips, feet straight or slightly bowed inwards, put you hands on your hips, arch your back and, keeping your hand on hips, back arched, inhale slowly and then bend down while exhaling slowly in a continual motion. Once your down to the lowest position you can get comfortably, inhale slowly and then while exhaling slowly bring you hand from your hips so that they are reaching towards the floor, again in one continual motion. Now, here's where it gets tricky if you haven't done this before. Inhale slowly and while exhaling, slowly force your head, not your hand!, to the floor, keeping your legs and knees straight, and forcing your butt towards the ceiling, trying to keep you back also arched at the same time. Don't push off with your feet. Use your feet for balance but all the pushing should be coming from your calves, your thighs, and by arching your back and forcing you head towards the floor. Do not force it til you are familiar with this technique. If you are doing this correctly, you will notice tension in your hip joint tendons and your sacroiliac, possibly in other muscles as well. You want to release the tension in those muscles by means of breathing slowly and regularly, releasing some of your stretch towards the floor on the inhale, stretching on your inhale. When coming out of this position, keep your back arched and put your hand on you hips. The real beauty of this technique is you can cheat by keeping your legs slightly bent until you can actually straighten them all the way. Once you've gotten all the muscle groups relaxed you can then breath in and then exhale slowly and try to straighten out your legs. This also helps you actually feel the difference in the muscles in your back side that really need to loosen up to accomplish the stretch all the way to the floor. Those are the important muscles for this and the reason that you have to be careful when you come back up. Remember, inhale means loosen slighly on the stretch (relax mostly), exhale means s t r e t c h but also pay attention to loosening and getting oxygen to your hip and sacroiliac by loosening there while at the same time forcing you head towards the floor with back slightly arched.

Fourth. When straightening up, keep your back arched and keep hands on hip. If you have back problems, this is especially important. And if you don't have back problems, you don't want to develop them so remember to do this if you are stretching towards the floor for any length of time before you come back up. If you are in reasonably good shape and take care you probably won't strain your back too much.

You can dooo it!

I just tried this and I can touch my finger tips to the floor on the first try. I cheated with the slightly bent knees and the slowly inhaled to tension and then exhaled when forcing my back to arch and my head towards the floor, back arched slightly. Can't put my hands flat though--I used to be able to do that. I can put four fingers from each palm and into an arched palm position though. I don't remember but I'm guessing that's the intermediate position towards flat palms on floor.

I don't know much about the head to knee thing as I never could do that anyways. Nor splits, not fully. If you are a women you eventually should be able to touch your head to your knees but it will likely take some constant practice to get there over several weeks at minimum would be my guess. When I was fully doing my stretches, I did a 15 minute routing in the morning before I took a shower and a 30 minute routing (non-cardio) before I went to bed. That was almost two decades ago. I think it paid of quite well for me as my bake is much straighter than it was in my early 20s and it has remained strong ever since I took up yoga, even though I don't practice yoga very often now.