Mitred corners give a really neat finish to the corners of a vent or split in a skirt or jacket. You can also use them as a decorative yet clean way to finish off a split or notched hem.
If the inside of the garment is going to be hidden by a lining you can leave the edges of the mitre raw and unfinished. But if not you can neaten the edges of the hems first, usually with overlocking or another way of finishing like a simple zigzag stitch. Or you can double turn the hem by just 1cm so it can be hand sewn or blind hemmed later to finish the hem.
1. Press up the hem allowance. (If double turning press up 1cm first then press up the hem). Mark on the vertical seam allowance of the garment where the folded edge of the hem sits. You can mark it with chalk or a fabric pen.
2. Fold over the vertical seam allowance, press that in place and mark where this comes to on the horizontal edge of the hem, again using chalk or a fabric pen.
3. Unfold the vertical seam allowance and mark where the pressing lines intersect. This point will become the corner of the hem.
4. With right sides together fold the garment so the two marks on the edges of the hem come together and the dot on the intersecting pressing lines is on the new fold. (If double turning keep the 1cm fold folded over.)
5. Sew from the dot on the fold to the edge marks. Backstitch at the start and finish of your sewing.
6. Trim the excess fabric down to 6mm and press the seam allowance open and flat.
7. Turn the corner to the right side making sure the corner is nice and sharp. Use a point turner to help to get a sharp point, but be careful not to poke through the corner!
8. Press the whole corner nice and flat. You can use a pressing cloth and plenty of steam if you need to.