Okay so you build the turtle deck base and bevel it as per the plans, then line up the two score cuts on the turtle deck piece, with the edged of the turtle deck base. Next bend and glue the turtle deck to the turtle deck base, there should be excess foam that you trim off so that it's flush with the bottom of the turtle deck base.
That's the best I can do without pictures, let me know if you understood that. If not I can whip one up real quick and take some pics along the way.