One tip I suggest to folks when they're working across the fold is to first fold whatever papers or images that they're going to be placing in that area in half before they glue. Also watch to make sure that when you're gluing your paper, you cover every inch (I missed a couple of spots resulting in the above extra wrinkles. Ce la vie.)
I am teaching at The Making Place in Escondido this Saturday. This is my last San Diego class for awhile (I don't have any on the schedule for May through July. My June and July schedules are full with out of state teaching.) Catch them while you can!