Drawing and painting skulls should not be difficult. Like most other freehand elements, if you break it down into a few simple overlapping shapes, you'll have great looking skulls in no time at all. Here's the method I use to replicate the Forge World style skulls you see on their resin bits.
You could easily modify the shape of the skull here to suit your tastes.
How to break a skull down into separate shapes
The first thing you need to do is take a good look at the area you're working in. Once it's ready to go (meaning you're at the point of adding your freehand work), you have to imagine a line across the middle of the available painting area. This is where you're going to start. You don't need to draw the line on your model, just remember it for the first part of the skull. From that, you'll build on it and your scale shouldn't change.
The skull is broken down into 5 parts. The head, nose, eyes, cheekbones and jaw. We're going to start with the head and add those other parts on top of it in a particular order so that we "build" our skull and it stays symmetrical and balanced.
The head is nothing more than a simple circle. It should extend slightly below that imaginary line we have across the middle of our work area.
The nose is made up of two small triangles that are placed at the bottom of the circle in the middle.
The eyes are two more triangles placed one on each side of the nose along the bottom of the circle as well.
The cheekbones are maybe the hardest part of this whole thing since they are not an exact shape like a circle or triangle but more like protrusions out from each side of the head. They stick out at an angle from each eye. You can make them any shape you want really.
And the jaw. It's an extension of the head that comes down slightly and is angled. Think of it as a square that is resting on one if its corners.
Adding a bit more detail to your skull
Once you have the basic shapes blocked in, you can go back and do a little highlighting and shading to give the skull some texture and depth if you want. Of course you can always leave it simple and one color as though it's been stenciled onto the vehicle as a kill marking or such.
Make sure to check out these posts as they might help:
How to draw Librarian heraldry