Okay, this might seem funny, but I took some 2X4s and cut them down to 2X3s. The reason for this, is because we are putting 1/2 inch Medium Density Fiberboard(MDF) on both sides of the 2X3s so it will be flush with the drywall. I cut them the length of how tall my arch would be. I did not get a picture of this, however, you just nail or screw them to the existing 2X4s right under the header.