I'm still a little at war with embroidery, but over time I've learned a thing or two through "learning by doing". Maybe the tips will help you a little bit :o)
Here I show you how to embroider mouth and eyes. To do this, I first mark the spots where the eyes are supposed to be later, with pins. In my case I have now embroidered somewhat longer eyes, the distances can of course also be shorter.
Then look for a place where you can insert as invisible as possible (in this case this is on top, because there will be hair on it later). Then look for a place where you can insert as invisible as possible (in this case this is on top, because there will be hair on it later). However, the starting point is usually (almost) not visible anyway (e. g. at the back of the neck would also be a good spot).
First go to one of the pins, which one doesn't really matter. Then insert the needle at the second pin to form the first eye and come out at the third needle (the first of the second eye). Now insert into the position of the last pin and return to the starting point (exactly the same space between the stitches where the thread first went in). Now fix the threads with 2-3 knots (Attention: do not pull too tight, the thread should only be tensioned normally, so that the threads of the eyes do not hang around loosely, but lie nicely against the crochet piece - I would not sculpt the face with the eye thread directly). Finally, hide the threads inside the head and cut them close to the exit point.
For this eyes I've used thicker yarn, if you have thinner yarn, you have to embroider more often over the same eyes, so that several threads lie on top of each other. For mouth and eyebrows I always use thin thread (embroidery yarn or other very thin wool, it even works with sewing thread).
I also mark the mouth with pins and test the shape by putting the thread around the needles. Then, as with the eyes, insert in an unobtrusive place and come out at the first pin.
Embroider the mouth with simple stitches. The distance can be made larger or smaller, this also depends a little on the shape of the mouth. Here I have chosen quite large distances. When you have finished the last stitch, you have to do the finishing of the mouth which makes it nicer, insert the needle under the first section and stitch around this part 2-4 time, always come from the same side, so that you "wrap" the yarn around this part of the mouth.
Repeat this with the other sections until you reach the other side of the mouth. Now go back to the initial point, knot well and hide the threads inside the head.
If you want to embroider eyebrows, simply do this before returning the thread to the injection point. The face is ready now :o)