When I have settled on a thumbnail composition, I make a rough drawing like this one.
Next, I make the “final” line drawings. They are mostly drawn separately and layered on top of each other so that I can move things around until they look just right to me. (The rectangular lines around drawing are guides show the edge of the book and safe zones for text.)
Then, on the computer, I fill in flat colours under the line layers. Usually, it takes some experimenting before I am happy with the colours.
After that, I create some shade, shadows and highlights. These help add some dimension to the pictures.
At some point, I apply some texture that I have scanned in, and add extra colour here and there.
I also use coloured pencils, textured paper, watercolour, etc. to scan in and layer onto the images. Here, I used coloured pencils for texture and pattern, and some striped brown paper for the Duke’s suit.
Finally, here is what the illustration looks like with all the parts together!