I have a tricky bed.  It's from the 1930s and is 9 inches less wide than regular twin mattresses.  I figured that an elastic bed skirt might be the answer....and IT was.  I used upholstery T-pins to shape and stretch it around the edges of my bed.  The T-pins are essential to this process to keep skit in place and to manage the stretching if you are trying to do it alone as I was.  Looks great!  I'm happy.  PS: the upholstery T-pins can easily be ordered online if you don't have a local source.