Lay your corner that you wish to mitre on your table right side up, then fold it diagonally right sides together so that the cut edges meet.
You will have stitched through four layers of the hem allowance. To reduce bulk in the corner, trim the excess seam allowance away diagonally, to within 2mm of your stitching.
Finger press your tiny seam allowance open, and turn hem through to the wrong side. It should now look like this:
Repeat this for all corners that you need to mitre. It is best not to press your corner until after the hem is sewn. Start sewing your double folded hem, and as you approach the corner ensure your mitre sits flat and square.
Stitch to the inside corner of the mitre, I use my handwheel to ensure the needle is placed in the exact spot. With the needle down, raise the presser foot and spin your work 90 degrees to continue the hem, ensuring the corner lays flat and square.
Once your hem is completed you can give it a press.
Voila! Perfect corners - beautiful on the outside and the inside!
I hope you find this useful, and if there are any questions I will answer in the comments. Happy sewing!