if you are using a 1/4" seam to attach the binding to the first side- when nearing a corner stop- place a dot (pencil works) 1/4" from the corner on the binding fabric- then stitch right to that dot- stop- raise the pressure foot (with the needle down) turn the quilt so you can stitch a diagonal line off the quilt/binding...stop, cut threads, remove quilt. fold binding fabric straight up- then back down-lining up the raw edges again- start sewing the second side-starting at the 1/4" dot...continue around the quilt stopping & folding each corner-
now- when you fold the binding to the other side to stitch down when you reach a corner fold one side straight - then fold the second side forming your miter.]
try it on a small practice piece (like a pot holder) to get the hang of it