My method is best explained visually;
I start with a regular photo.
I digitally invert the colors to get a negative, at this point the picture resembles a film negative.
I simply paint what I see on this inverted photo. Sometimes I even do it "blind" and guess what the inverted colors would be. Helps to have a color wheel memorized.
This is the final print. I photograph the painting and then invert the colors again to get this.