Stitch the seam, pivoting the stitching at the corner. Cut across the corner of the seam allowance, close to, but NOT through, the stitches.
Turn to right side. Use a chopstick or eraser end of a pencil to carefully push out the corner from the inside. (If you push too hard, you can make a hole in the corner.)
Press edges flat. The finished corner has less fabric inside and looks good from both sides.