gjunior, when you draw with a pencil, do you trace or draw your lines in a single pass of the pencil, or do you sketch your drawings, using many passes of the pencil to build up the shades and textures?
That's primarily what the "layers thing" is in airbrushing. Rather than trying to get the perfect line in a single pass of the airbrush, with the width, shape, color, density, and texture to be perfect in that one pass, we use multiple strokes or passes with the airbrush, using minute amounts of paint to build up the painting to the desired look. It's much more forgiving, just like the pencil in that if you tried to get everything perfect in a single stroke, you would go through a lot of paper, erasers, or whiteout/eraser fluid trying to do a whole drawing using that method.
For those struggling with the lines, etc.
First, be patient with yourself, this is not life or death. Relax and enjoy the ride, the learning and discovery is a great part of the enjoyment.
You don't have to have a death-grip on your airbrush, for many, that is one of their biggest obstacles, they tense up so much, and go super slow to make sure they get everything perfect, that that becomes the problem in itself. Your muscles clench, and your hand and arm shake because of the tension, and going slow means that every shake of your hand, arm and body, including your heartbeat, has a chance to show itself in your paint stroke. Add to that the fact that the paint and airbrush don't know anything about going fast or slow, so by going slow, you end up spraying a whole lot more paint in that little stretch of shaky lines, and you get thickesh wiggly lines because of it.
I don't know how you hold your airbrush currently, but you might look at the photos I posted here. These show a grip or hold that lets your hand and fingers stay in a more relaxed pose or hold, and that lets you get better control and smoother lines.
Water trap at the AB for an Iwata
Depending upon the model brush you use, you might find that using a grip on it would help you. I know Grex makes a grip that will work on the majority of gravity feed brushes out there, not just theirs, it gives you a pistol style grip that keeps you from working in a fist-like hold on your brush, there may be others out in the market too.
I have put them on Iwatas and other makes for students who have a struggle with gripping the airbrush, and using it for any extended period of time.
I modified one of mine to make it more comfortable for my hands (I have large hands), and was able to tweak it, shape-wise, quite easily with a pair of nippers/cutters and an emery board to smooth out any edges left by cutting the plastic grip.
All this may be more than you wanted to know, but at least you have the choice of reading it or not.
Good luck guys, practice 'til you can do the drills without having to stop to think about how to do them. Work safe, this is the only body you get, so you need to be careful with it and the environment.
And last but not least, relax and have fun.