First you'll need a skirt that is too big and a new piece of elastic. 'New' is flexible. I hi-jacked this piece from an old pair of skivvies! SCANDALOUS!
Flip the skirt to the inside of the waist and find the side seam.
Carefully, and being sure to stay between the top and bottom seams, cut a small hole next to the side seam, then pull out and snip the elastic as well, but don't pull the elastic out of the skirt because we're going to use that to cheat the Craft and Sewing gods.
Take your new piece of elastic and safety in it to one side of your old one, then carefully tuck the pin and the attached pieces of elastic back into the hole.
Then, take the other, non-pinned side of old elastic and, while holding the free tail of the new piece, yank the old elastic through the tube. This should automatically thread your new piece in as you pull the old one out! TAKE THAT, SEWING AND CRAFT GODS!
Once you have the elastic threaded through the tube, un-pin and discard the old elastic, then stitch the ends of the new elastic together. Be sure to go over it several times so that it has a lot of hold. No one wants a wardrobe malfunction. We in the real world aren't looking for publicity.
Once that's done, stitch your little hole back closed and that's it!
See? No difference at all from the outside, only one tiny stitched-up place INSIDE and now we have a new skirt that fits like a dream! YAAAAY! :)
Listening to: "Raven" - Mejibray