Hopefully you are at this point and if not you can return to this post when you are. Trim the edge of your mat even with the top edge, cutting away the batting and the backing.
Take the quilt out of your machine and fold the binding towards the top and back down along the edge of the quilt. Start your back stitch 1/4″ from the edge and sew along the edge to the next corner where you repeat the step.
When you reach the final side, stop sewing a few inches from the loose end where you started. Overlap the ends and trim the long end to overlap the number of inches that you cut your binding which in this case is 1-1/2″. Join the 2 ends with a right angle seam, trim, press seam open and stitch the remaining binding. You will see that it is the exact length. (Here is where I got interrupted and forgot to continue taking step by step pictures! Hope you can follow this last step.)