well, it does gather up a bit, you just smooth it out as you go. The other way to do this is to sew the casing of the pants first. (fold down the waistband wide enough to put your elastic throught) and sew it down, leaving an opening near one seam. Then, pin a safety pin to one end of the elastic and run it through the casing until you get back to the opening. Securely sew the two ends of the elastic together, then sew the casing opening closed.
I hope that made sense- I can try again if it doesn't.
did I say one project at a time?