Okay! You've made lovely bound buttonholes on your coat, and now you need to finish the backs of them (on the facing) so that you can actually put buttons through them. Unfortunately, it's kind of hard to see very well with my fabric, but hopefully this will still make sense to you. Here are the steps:
1. Position the coat so that the buttonhole front is facing you. Mark the buttonhole rectangle on the facing side of the fabric by sticking a pin through each corner. My pins are indicated by blue arrows below:
4. Cut carefully through the center of the rectangle, and out to the four corners, being careful to only cut the facing fabric. This forms little triangles on each side of the rectangle.