Stop sewing 1/4 from the corner; fold your binding strip with 90 degree angles and tuck under your already sewn side. Start sewing next edge 1/4" away from corner. Repeat with all 4 sides. When you are ready to flip to the back and sew down be sure to clip your quilt corners at a 45 degree angle to reduce bulk.
I don't normally have issues with the miters but if they are being particularly cantankerous, put a pin through the miter until you've gotten the back sewn down. If need be, put a stitch or two in the miter to hold in place.