Make sure the plain side is being pencil marked on. Measure 5 7/8th inches from the edge of the spine, not the middle of the spine. Do this on each side of the spine, and mark on the top and bottom so you have a straight line on each side. That line makes it so you can bend it or score it (I recommend using the score method). In the picture the right side of the album was bent and looks bad, while the left was scored, looks better and was easier to fold .
When the flaps(illustrations) are folded inward, the front cover and back cover ,should measure 5 7/8 inches.
Each flap(illustration) should measure 5 1/2 inches.