I started by drawing a rough outline of the penguin, and carefully closed all the holes in the shape. I next filled in the spaces left by the outline with black, and continued shaping it using different sized crisp* brushes. Once had the initial image done in black and white, I laid out the beak and feet in approximately the right places and filled in their outlines using the same techniques.
I continued by adding the initial shading. Applying a small crisp brush with slightly darker yellows to the edges of the beak and feet caused the image to start taking on depth. Continuing this process with several shades of yellow and grey I managed to block in the shape of the shadows that I wanted.
Whenever working on small areas or doing careful work, I find it extremely useful to open another view of the image I am working on. I zoom in on one window and keep the other unzoomed. Then I arrange the windows so that I can see them both simultaneously. Now I am able to draw in the zoomed window where a small slip of the mouse won't mean disaster**, but also able to see how the work I am doing looks at its actual size and resolution.
Once I had the shape and primary shading done to a level I liked I brought out the convolve tool. The convolve tool is very useful when drawing things in the GIMP. It allows you do hand anti-aliasing, controlled smoothing, and a host of other neat effects. I used the convolve tool set on blur with several different brush sizes sizes to smooth out the shading. I also used the air-brush extensively to lighten or darken areas that I had smoothed a little too flat.
Essentially I applied those techniques over and over again until I was satisfied with the results.
Finally to create the black and white and three color versions I used the Threshold plug-in available from the * plug-in registry to create a thresholded image that I then touched up by hand to smooth out the flow of the lines and to add hints of detail.
*Larry Ewing <email@example.com>