I decided to show how I work, especially when I have line drawings available. Otherwise, I do it by hand, pixel for pixel!
First, I mark the lines with a highly visible color:
Then I start to color the such that I get out all the odd pixels I do not want:
Pre-colored with a darker blue color such that the contrast between the blue and red is more visible. so I easily can see that I have removed all the odd pixels.
Now I have removed the blue color and made red to black (left-click on the red color, then right click on the black color. Then use the eraser and keep in the right mouse button and begin to move over the drawing with it.). so now I'm ready to start the real work, to get all the details correct.