Since the two sides and the bottom don't need to be hemmed, use the serger for those, and even serge the top edge to stabilize it and to prevent having to do that small "turn-under" to hide raw edges....that way you only have to do one wide hem for the top edge....and it's the shortest measurement (course - if it's a king size sheet you've still got a long stretch to do). As for that wide hem, I'd just make sure that the fabric is laying flat and unstretched when I turn it over..put the pins in across/perpendicular to the seamline, not parallel to it, and if you want to stabilize the fabric for the stitching, lay down a strip of blue painters along, but no on, the seam line = this will keep the fabric from stretching too much when you sew it...and I'd sew it with a small zigzag to build in some stretch to the seamline. Good luck.