I use double fold binding. It does help to press the binding and corner after stitching it to the top before folding it over. This sets the corner 45º fold and makes turning the binding to the back easier. Once you have it turned to the back, pin the binding to the back of the quilt about 3 inches from the corner. Use a stilletto or narrow object (like a darning needle) to smooth the binding up to the corner and hold the binding down along the adjacent edge and fold over the other binding edge over the first edge. Leave the needle/stilleto in there until you get the correct 45º on the binding edge that is on top. When it's perfect, place a pin in it and draw out the needle/stilletto.