The process varies on what you're working on.
You can afford to go nuts on the canvas when doing something you're used to drawing, but that's not guaranteed to get you good results. It could work out, though you might need to re-draw the thing several times. It's not exactly a bad thing—you might learn something you wouldn't have otherwise—but you can at least try to save yourself some time and give it some proper thought. Plus if you're not drawing anything you're not used to, you probably won't learn anything by diving head-first.
The more time you spend contemplating on it, the less time you'll spend reconsidering things as you're drawing/carving/painting a piece, and you might only need to do it once or twice.
For example, suppose I want to do a dynamic shot of a girl's underwear as a gust of wind lifts up her skirt just when she's about to pick up a penny off the ground. I know how to draw girls as well as underwear, but that dynamic shot will pose a problem in perspective and anatomy. It'd be a really terrible bitch if I was shading in the stripes when all of a sudden I realize this girl's arm is three times longer than her body.
If I was really eager I'd just try to sketch it up as soon as possible, placing guidelines for the body all over the place and make sure I put that ass between the bottom right corner and dead center. If I wanted to be a bit more careful—which I rarely am—I'd review some perspective, recall what a vanishing point is, and then review some female anatomy.
Once all the necessary things are in my head, I try to jot some of these down somewhere to make sure I keep them in mind—things like texture, color, lighting, angle, depth, etc. Afterwards it's a matter of
>guidelines and contours
Here I sketch in the basic structure or skeleton of the piece. I take care to stroke as lightly as possible, using an HB or 2B pencil. If necessary, I set up things such as vanishing points, marks for certain features (windows, stripes, facial), and crosshatched areas for shading later on.
>features and details
Here I trace over the sketched shapes using greater force, usually with a 2B pencil. I try to only stroke once over a line, so as to not make it cluttered and unclean.
>shading, lighting, and color
Finally, I start filling in tones, from darkest to lightest. In cases where there'd be clothing or rock, I try to mimic the texture. If I plan to color the thing, instead I fill in and work on each color one by one.
It's essential for me to complete the piece in my mind first before putting it on paper. If I haven't thought it through completely, hoping that the piece finishes itself, I might end up regretting something when I'm sticking it into the scanner (or a few days later)
