You like process gifs, I like getting support messages like yours.
That sort of film is called frisket film, and you should be able to find it at most art stores, at least at those who sell airbrushes too. Frisket film is used to mask the areas you do not want to cover with paint, and yes, it is usually cut directly over the drawing surface. Basically you take a piece of film, place it over the drawing and then cut it out in appropiate shapes. Keep in mind that frisket film is a low-tack adhesive so it won’t damage the drawing below. When cutting, you just have to apply the correct level of pressure with your X-acto knife as to avoid cutting through your art. It may sound tricky but it’s not. One important thing, though: replace your blades frecuently (and I mean very frecuently). If you use worn-out blades you’ll apply more pressure than needed and the surface below will be easily damaged.
Everything about the frisket film technique is actually extremelly easy; it’s all about carefulness and patience. Most of the difficulties with airbrushing have to do instead with getting the proper paint thickness or the right air pressure… things like that have a dramatic impact on the quality of the final work, but can only be properly learned -at least in my experience- by constant trial and error.
Set yourselves as much as possible against all machine-work (this to all men). But if you have to design for machine-work, at least let your design show clearly what it is. Make it mechanical with a vengeance, at the same time as simple as possible. Don’t try, for instance, to make a printed plate look like a hand-painted one; make it something which no one would try to do if he were painting by hand, if your market drives you into printed plates: I don’t see the use of them myself.
William Morris. Art & Beauty of the Earth.
Beware. Each time you add some cool textures to you digital drawing to make it look more organic, you are making the ghost of William Morris angrier and angrier.