some Yonex shorts have a single loop around the waist, which I can't tie a butterfly with it. how do i tighten such a shorts?
You mean there is a string around the waistbound but it is all in one piece? If all you see is a loop, why not just pull and tie a half-hitch? Or cut the loop and you can tie your bow.
?Honestly? Ok, the Boys Scouts aren't doing there job for today's youth. Here you go: http://iwillknot.com/slip_knot/