1. I usually start off by roughing my layout with blue pencil. I then darken those lines with a hard pencil so it will be picked up fairly well by my scanner, everything is still pretty rough at this point. I scan the lines in at 300dpi, set my artboard in Illustrator for the final output size.
2. I place my scan into Illustrator and lock that layer down. On a separate layer, I use the brush tool and start going over the lines. I always mess with the settings to get a nice taper on my strokes and use the pen tool for those areas that the brush tool just doesnt look good on. The great thing about vector work is the ability to adjust the lines once they are down. Using that advantage, I tighten up the design and add anything I may have left out of the rough sketch.
3. I create another layer under my lines, and using the pen tool, I quickly fill in my flats. As long as you split the black lines you should be ok. If I'm in a rush I use a totally different method for this. I'll discuss that one some other time...
4. I create another layer between my lines and flats and start to add the shades and highlights. On more elaborate designs, I split the shades and highs on different layers to have a bit more control using transparency etc.
5. Once I'm happy with step 4, I go back to my flats layer, duplicate it, and start filling in the gradients and textures as needed. I've created a number of texture patterns that I always use for different areas such as brushed metal, dirt etc. When I'm done with that, I copy my layers into Photoshop using the same final file size @ 300dpi, or whatever is requested. Since its all vector at this point, I can scale it up to any size needed with no pixelation. I start to add the effects that Photoshop just handles better than Illustrator.
6. I add the final polish to the design in Photoshop, crazy burst backgrounds are so much faster and less file heavy in PS. I drop my name in and wha-lah! The file is uploaded to a secure FTP to be downloaded...
This design took me a few hours from start to finish, but that really varies on each design.