check the tutorials there are good instructions for mitering bindings; is it the front or the back you are having a problem with? if it is the front you may need to do some un-sewing. when stitching the binding to the front you have to stop right on the 1/4" spot of the corner; take the quilt out, turn it, fold the binding up, then straight down, line up the edge with the raw edges and start stitching again to the next corner (1/4" from the corner in both directions.
if the miter is a problem on the back and you have a whole 1" to work with you can clip excess if you need to but you should be able to fold a good miter with that much fabric to work with.