Gathering may be accomplished by machine basting a straight line about a quarter inch from the edge of the fabric, holding the threads that are at the end of your baste and gently pulling the top thread while keeping the bottom thread steady, which can also be accomplished by a sewing technique called casing, which results in a little case through which you can pull your shoelaces or ribbons.
Well, since I put it that way, I almost look like a regular seamstress.