Amy Alan answered:
Thank you! You close it with velcro straps. It took a few extra brain cells to figure out, but I decided that the front shoulder straps (wait- do dogs have shoulders?) would attach to the front of the underbelly strap with velcro. I attach each side individually. Then, the underbelly piece comes up and velcros onto the side of the coat, so that at least some of his belly stays dry. If you look at the photo of the pattern piece, you can see my marks for velcro, which may help make sense of the pattern piece.