If I know I'm heading somewhere I think will be particularly interesting to draw, I make a point of taking my sketchpad, but I will generally have a go at drawing whatever happens to be around me whenever I have the time and inclination to do a sketch. I usually spend between five and twenty-five minutes producing the sketch, using pencils on paper.
I scan the sketches into the computer at high resolution and then get to work with my graphics tablet using Photoshop. Without wishing to entirely demystify my working method, I overlay many different layers of colour to different parts of the drawing, adjusting the levels of the scanned image to alter the level of contrast, and often invert parts of the image to show negative white on black lines.