Hey
Nice work HladekStepan
One thing I might suggest is when you draw a head, skull or anything else, to try to see it as a 3D object. Try to create mass, almost as if it can be touched. It is up to us artists to create the illusion towards the viewers they are looking at a real object/person, not just a sketch on paper or (digital) canvas, which is only 2D
Why not draw a few real life objects as an exercise? Use traditional media if you can. Pencil and paper is ideal for this. Carefully observe, then simulate where you want your line to be drawn a few times, then draw the line and move on to the next part of the object you want to draw. This will show you what I am trying to explain above.
you’re doing great, keep going!
edit: forgot to mention… I think you would benefit from doing some perspective exercises as well… think about it… no work/studies you do are a waste of time… they all come in handy later on… 















