To do a mitered binding...
When you come to the end of a side, stop exactly ¼" from the end. Backstitch. Then turn the quilt around so that your next side is ready to sew. Fold the binding straight up to form a 45 degree angleat the bottom of the fold. The folded side ofa double binding should be on your left, and the raw side on the right. The folded up binding should make a bottom fold that is line with the diagonal on your quilt. Place a small pin to hold it in that position. Take the rest of that long binding strip that's been facing up, and fold it down over this other 45 degree fold and line the right (raw) edge up to your quilt edge and begin sewing again. Backstitch. Then sew down that side until you come to within ¼" of the next side and repeat the above process. Hope this helps. :)