My input, and I could be totally wrong, but this looks a bit rushed through the steps trying to get to the result. If you don't already, before getting into the studies proper, take ten minutes and a piece of scrap paper (anything will do), and practice ellipses, circles, straight lines, c- and s-curves. Vary lengths, direction, etc. If you're not familiar with the concept of "ghosting" lines, look it up, (proko's drawing basics is a fantastic resource). In terms of what's there, it looks like a view from below, so I'd expect to see more of the underside of the jaw and the connection to the neck tube. At each set of stages, (he breaks down the steps into logical groups that make sense), pause and reflect on what you've done, correct anything that looks off before moving on. As we build on top of prior steps, if the early one's aren't right, the later ones won't be either. Good luck and keep practising! Several shorter studies with reflection between/after are better than one long one.

This is due to perspective. Just like how you can divide planes into halves using the x-method(shown here: https://www.proko.com/course-lesson/the-x-and-x-ray-tricks/ ) - there are methods to divide planes into any equal portions that you need. Here is how you'd divide it by thirds: https://web.archive.org/web/20160730031252/http://andreasaronsson.com/blog/wp-content/uploads/2012/05/DivEquParts_03.png The image is from this blog(recovered using web.archive, since the blog seems to have fallen off the internet). The blog shows how to divide planes all the way up to 1/27th if you are interested. https://web.archive.org/web/20211017170937/https://andreasaronsson.com/guides/perspective-drawing/divide-into-equal-parts/ But really, what you'd do for a "rule of thumb" is practice training your intuition a bit. Draw a plane, guess at where the thirds would be in perspective, then use this method to check how well you guessed. Do that enough and you'll start to get a feel for things. Honestly it doesn't have to be super accurate - just accurate enough. If you are always drawing from observation then you'd just rely on the proportions you can see.

My advice: after you do a set of steps, say 1-4, 5 and 6, and then 7, and 8, pause and digest what you've just done. The sooner you solve the issue, the better. Is it an extreme pose or something more readable? What steps were easy? What steps were hard? In a linear process like this, where stuff builds on the previous, if the early stuff isn't quite right the rest will be more difficult or might not feel quite right if you just keep pushing. I like to begin by looking at the reference for the overall orientation and sometimes do a cylinder to help orient. Then as I dip into the construction, before I move on to a next major phase I pause for just a moment to see if anything is glaringly wrong, and then move on. I try not to dwell and if I miss something, I miss something, but I try to ask where I feel weakest. Sometimes I don't see it till I step away for a bit. I struggle with views from below, seeing lots of the underside of the jaw. So I aim to do more of those. I also often draw along with the critiques, making notes, and try to apply it immediately to my studies.

Not in addition to the steps/lecture. Just be patient with it. It takes a lot of time to get comfortable with it all.

i would say that the steps demonstrated are a great way to practice simplifying the complex subject of drawing the head, not sure if you have his other class but maybe if you looked up some basic form intersection and modification exercises of cube, sphere, pyramid, cone, etc. could be beneficial for drawing everything