Next you are going to glue down the arch guides along two sides, just along the bottom edge of the top molding, on top of the red wall. Tape the arches together once you have dry-fitted them for two sides. This photo is of the finished arches, but you will be using the big pieces of the guide arches, plus one small part of an arch to fit them to the back support neatly. It is important to try and not have cut edges showing at the corners. You will notice here that I have a tiny sliver of box showing at the near corner, which will be touched up with paint. This happened because I forgot to check the dry fit before I glued it down. You can't even see the mistake in the finished piece. Decoupage can be forgiving if you finish it well.