This is the way I do a mitred corner.
I'm not saying it's the only way, but it is the way that I get perfect mitres every time.
You can Google or YouTube various other methods.
I have found this to be the best for what I do.
Please see NOTE at end of this post.
Sew your ¼” seam, stop a ¼” from the end (corner).
Remove from the machine, fold fabric tail at a 90 degree angle back along the raw edge of the quilt, this will create a triangular fold of 45 degrees.
Stopping an accurate 1/4” from the end will help with 45° angle on the corner when you start sewing again....it gives a definite “feel” of where to start sewing again. It will also give an accurate mitre on both sides of your quilt binding, when done properly,
Start sewing again ¼” from the top edge and with a ¼” seam. When you finish, fold the fabric over to the back and slip stitch into place, you will have this little fold again, push one side of the fold under the other and you will have a 45 degree angle. Pin securely and use a couple of stitches to hold the fabric down properly.
click on the photo to view full size and see all of the instructions
I didn't invent this method,
I don't know who did,
but after doing bindings with mitred corners in various ways over the years, I have found this one to be the most successful to obtain a perfect mitred corner every time on both sides of the item.
This is the way I do the mitred corner and is a tutorial to show just that....
I am not claiming this method as my own, so I am not infringing copyright or making claim this method is mine and mine alone, however the photos used are mine and shall remain so.