I'm left-handed and there are very few places on the Internet where you can see photos of how we left-handers do things! So this is special for you!!
Note that I'm sewing shut the folds on both the front and the back of the quilt at the corner. An important step.
This technique is described completely in my book, "Binding Your Best", with Bonnie Craig. Easymade Publications.
Approaching the corner - sew past the line of machine stitching
you used to secure the binding to the front.
Now, fold over the new side, make a perfect miter and a perfect corner.
Secure with a couple of stitches.
Sew shut that fold!
Ladder stitch or whipstitch.
Take your stitching to the very point!
Flip it over to the front and sew shut the fold; use a ladder stitch for invisibility.
Poke the needle thru to the back and bring your thread to the back again;
continue on the new side!
Shown here using Valdani thread!