When I started with the ears, I thought I could fake it by drawing a circle á là Mickey Mouse. This didn’t work out so well.
I therefore turned to tracing again. Because the image was so dark, there were two places that could be the top edge of the ear. I went with the one closest to the center under the justification that rounder is cuter. However, there was still something that bothered me about the result.
Going back to the original, I noticed what it was: the ear bends over at the top. Since I was working on the computer, the right thing to do would’ve been to add a new shape that composed just the top of the ear. However, due to the translucency ears naturally have, the top is usually the same color as the inside, so I played lazy and only added a line.
I therefore drew the whiskers first. For this, I used long, thin rectangles, which I rotated to match each of the white lines in the photo. I was unable to see where they ended, though, so I made them approximately as long as they were on the white mouse. The fact that they reached as far as the eye without covering it was a happy accident. For the color, I chose a light gray and set it to 50% transparency (because white looked too much like someone had added chalk lines to the drawing after the fact).
To simulate the lack of a clear-cut border, I removed the outline and set the fill to a gradient. The last piece missing was the mouth; however, it was covered entirely by the mouse’s fur, so I had no idea where it was. I therefore was free to do whatever I wanted with it.
There was one more thing left to do: the neck. Mice necks go practically unnoticed because they are so short and so wide, but since I was going to draw a mouse on his hind legs, it was of primordial importance that I had a neck and knew where it connected. As it turns out, the perfect circle includes some neck fat that isn’t there, and misses some of the fat that has to be there. I stretched the head shape to match better where the neck appeared to end in the photo. The result was rather weird, but it was what I needed to move on.
You’ll notice that there are two other things that are different in the drawing: first I replaced all the outline colors from yellow to a lighter tone of the fill color. I was using yellow to contrast against the black mouse; a lighter color gives the idea that the fur gets thinner and eventually fades into the background, without having to actually have it fade. Second, I rounded all the lines to make the mouse look more natural; I had been using straight lines to make it easier to use them as guides.