Un-zoom by clicking on the magnifying glass and clicking on your picture. Still using your line tool, draw a straight line from the edge of the first line you drew to the edge of the spine in a straight line.
If you are not satisfied with the way it looks, you can “undo” your work and modify it until you are satisfied. Once the page outline looks proportionate to your book, remove the excess at the edge of the pages by zooming in and using the eraser tool – make sure not to erase any of the back cover section.
Next we need to color the page part in a color unique to the rest of the cover. In this case RED is used but the choice is yours, as long as you use a unique color.