Picture 1: Shows lining up the 45 degree line on the top edge of the pieced top. The cutting edge of the ruler is extended 1/4" past the point of the top (seam allowance). I like to trim my backing fabric 2" from pieced top for a nice size/full hem.
Picture 2: What it looks like after slicing off the corner. Practice on some scraps first just so you don't get nervous!
Picture3: Fold backing fabric lining up cut edge with corner of pieced top on the fold. Place a pin in the middle. Now stitch from folded corner to the pin. Clip corner.
Picture 4: Turn corner right side out. I just use my finger to "pop" the corner. If needed, finger press seam open.
Picture 5: Sides folded under and pinned. Ready to be stitched down. When I do a hemmed quilt, I machine stitch down the hem. This would also look nice with a decorative stitch.
Picture 6: This is a picture of a Log Cabin quilt I did about 5 years ago. As you can see, the seam is still holding up. And it's nice and flat - no bulk.