Use this shim to elevate the outside edge of your trim molding before cutting it. Trim back the drywall with a sharp utility knife until the trim molding no longer rocks when it's set in place ...

For a 90-degree corner, set your saw to 45 degrees. Pro tip: On an inside corner, the top of the molding will be shorter. On an outside corner, the bottom will be shorter. To cut the left molding ...