Step 1: Start with a pencil sketch of the subject and scan it into your computer. Here is one I came up with just for fun:(actually, I sketched this in Painter itself, but that's the subject of another tutorial...)
Step 2: Clean up the pencil lines in Photoshop (or, if you're like me, in Painter). Here's a quick way to remove all those smudgy lines and get good contrast in Photoshop. Let's suppose this was what the original pencil drawing looked like (you used an HB lead and your sweaty hand got all over the paper):
In Photoshop, either select Image->Adjustments->Levels from the Menu bar or Levels from the Create New Fill button at the bottom of the Layers palette:
or....
In the Levels dialog box, make sure the Preview checkbox is selected (so you can see what the heck is going on) and click on the Set White Point icon (circled in red). We're going to use this to adjust the contrast in the pencil sketch. Notice that the cursor has changed to resemble the icon:
Move the cursor over an area in the pencil sketch layer that looks 'smudged' or gray. Click on it:
Everything at or below this level will be converted to white. Click OK. Voila! Instant contrast.
After some cleanup, here is what my 'pencil' sketch looks like. I'm now ready to start painting!