Insert a glass head pin on each side of the buttonhole to protect the edges from being cut. Starting in the middle insert the seam ripper and cut through the fabric.
Cut through the other side too. (Always use pins! If you think you won’t cut through the stitches, you will. I have!) Open the buttonhole to make sure it has a clean and fully open hole.
Starting in the middle use the fabric knife to cut through to the end. Then cut through the other side. The knife uses a pulling motion while the seam ripper uses a pushing motion. You can use the tool that works best for you.