check the tutorials on binding here on the forum they will help. but good measuring marking works well for me (well, 3 corners out of 4...i can never get all 4) anyway. put a little dot (which you can see) 1/4" from corner. stop stitching on that dot. cut thread, turn the quilt and binding fold the binding up, then straight back down and start stitching, down to the next 1/4" from the corner, repeat ... looking at the tutorial pics will help this make sense.
good luck, ive been quilting quite awhile now and like i said i can make 3 perfect corners ... for some reason...