Original Photoshop Design
You have to think of color separations like the "Transporter" from Star Trek. It takes molecules, breaks them apart and sends them to a destination where it puts them back together again. With color separation, you take a design and decide how many colors need to print on the press that will simulate the original image. This requires a little experience and an eye for color. This particular design, I decided was 9 colors. So the goal is to break the design down into those 9 colors then reassemble those colors when they print on the press to end up with the image on the shirt.
Below is the sequence that the design prints on the press and is very similar to how it looks as it's printing. These images are demonstrating how each plate looks individually as the design comes builds back together. (Click on Picture to enlarge)