This is a neat way to do the corners of your 'binding' when turning the backing to the front to use as binding. I had turned the back to the front before, but this way you really get a nice neat corner miter. I saw this method at a demo at a LQS. I didn't invent it. It has probably been around a while, but it's new to me.
Being careful not to cut your backing or front, trim the batting to desired width. Mine is 1/4" out from the edge of the quilt top. Trim the backing fabric twice the size needed to reach from the edge of the batting to where you want the 'binding' to end.
Fold the raw edge of the backing fabric up to the batting and press.
Fold again, this time folding over the edge of the batting and press. Do this on all four sides.
Open the backing back out.
Fold at a 45 degree angle at the corner, backing sides together.
With your ruler lined up at the bottom fold (notice I have turned the fold to the bottom in this pic), and the edge of the ruler on the batting corner point, draw a line from the fold up to the crease mark.
Opened out it should look like this. Your line doesn't have to be so dark! Refold and stitch on the line.
Trim 1/4" out from the stitching, continuing past the stitching, all the way to the edge of the fabric.
This is how it should look now.
Flip the binding to the front, over the corner.
Tuck under on the fold. Then stitch it down, using invisible hand stitches, or using a decorative stitch, if desired.
I made myself a little sample with backing and batting about 6" square to keep to remind me how to do this. I finished each corner to a different step, so I could look and remind myself how it's done.