I usually do it like Griz says just folding over twice, BUT if you want it o look really nice, I for one have to do one of two extra steps. If it is a tarp or something that will be stretched to show all the gathered places and mistakes, I Iron the hem once folding it then Iron it again folding a second time, OR I sew the first fold then fold and sew the second fold, THEN you have to go back and sew the hem again right next to the edge of the last fold if you want that double stitched look.
It may be wrong but I have to do one or the other. Mule
There are only two kinds of people that understand Marines: Marines and the enemy. Everyone else has a second-hand opinion.
Gen. William Thornson, U.S. Army