Here's a quick rundown of how I generally work on drawings, from sketch to final illustration. It's not always like this, mind you. Sometimes I just run with a drawing without thinking about where it may lead. In this case, however, the drawing was planned beforehand.
I do most of my drawing in Photoshop. I use Corel Painter on occasion, but I generally find Photoshop a lot quicker and easier to use.
So first, as you'd imagine, I start with a general sketch plotting out the composition, characters, and objects. At this point I'm not really thinking about the details, just the main shapes.
Next, I start inking the lines on a new layer. There's a lot of trial and error in this stage, playing around with the details. I think I spent the most amount of time getting the facial expressions right. The characters are all drawn free-hand, whereas I used the pen-path tool thingy to get the lines of the sleigh nice and clean. If you don't already know how, learn to use the pen tool. It really is a saviour when it comes to getting smooth shapes.
Then on a new layer underneath my linework, I grab a big brush and start filling in the colours. There's a lot of playing around here as well, to get the right mix of colour. I generally go for low-saturated pastels, but in this case, I needed a deep red for Santa and the sleigh.
This part's fun. Here's where the drawing really comes to life. First, I pick a direction for the light source, and then a grab a smaller brush and start blocking in the highlights and shadows. I'm not skilled enough with the airbrush to get smooth shading happening without it looking tacky. Instead, I go simple: for each base colour, I use one darker shade for the shadows, and one lighter shade for the highlights. It's up to you how dark and how light, but I generally find that subtle colour differences don't really work with my style. I don't really mind the "line" between one shade of colour and the next. I employ a lot of guesswork when it comes to shading, and while it's not always as accurate as it could be, it still brings a whole lot of form and life to the drawing. Getting the contours on the sleigh took some trial and error. I'm still not sure it's entirely convincing.
Now comes the finishing touches. Photoshop has a good variety of textured brushes, and they can be quite versatile with some practice. For the background, I picked a couple of lighter shades of blue and started sponging on some texture with a big "leaf" brush. Then I brought the size down a bit and the brightness up, and stuck in some tight clusters here and there. For the sleigh sparks, I grabbed a rough "star" shaped brush and a bright yellow, and started throwing them all over the place, alternating size as I go, so it comes out looking kind of random. Then with a thin brush and a dimmer yellow, I threw in some feint streaks along the sparks, to give some sense of movement. For a larger version of the final image, click here.
And that's it! Hope this was somewhat insightful. If not, you shouldn't expect so much of me.