Be sure to have perfect 90-degree corners on your quilt to start with by trimming before you add the binding. Then when attaching the first side of the binding sew to within 1/4" of the edge, lift your presser foot and pull the quilt away just a little bit while you fold up the binding away from you. You'll get a 45-degree bend in the binding going from bottom left to top right. Then fold the binding back down to fit along the right edge of your quilt, covering the 45-degree fold. Start sewing again with the 1/4" seam. When you're ready to do the back side of the binding fold it over to within 1/4" of the corner and bring the adjoining side down. It should make a natural 45-degree fold. If not - I use the point of my seam ripper to bring the binding closer to the edge while folding the top edge over it. YouTube has some good videos of the process if you're a visual learner. Good luck!