Mitered corner with a fold
Mark a line 1/2 inch (or less, depending on the room you have) to the inside from the edges. Then turn the corner to the inside along the corner mark. Now fold the other folded edges to the inside . You will get a mitered corner. Stitch the folded fabric in place with black thread at first (to hold it), then use white sewing thread to secure. Remove the black stitch when the white stitch is fully completed and tied off.