I do not trim off the excess batting and backing before starting. First I sew a narrow zigzag along the raw edge of the front. Then I use the walking foot to apply the stitching which has been lengthened to 2.6 or 2.8 on my Bernina 350. Then I trim out each corner - Patrick Lose's suggestion as noted earlier. Finally, I stitch the binding to the front, trim off the facing and batting, and hand sew the binding to the back. Been doing this since the mid 80's when I got my first walking foot. Have tried many times to master machine sewing on the last step, but have not been able to achieve anything but mediocre results - hence the hand stitching for the final step. My hat is off to those of you who can do it well!