Yet again, I bought a pair of shorts that were too tight in the waist because the larger size didn't fit as well everywhere else. I added a strip of fabric down the length of both side seams to widen them, which you can see above.
Here are the shorts before alteration. The shorts came with a belt / bow, in the same material. There was also a little hardware on the sides of the shorts to keep the hem rolled, but I removed it carefully before proceeding.
Here's how to let out a pair of shorts
2) (Optional) Serge or zigzag the edge of all the raw seams.
3) Take a strip of cloth and sew it down the sides of the rear part of your shorts, covering the serged edges. In this case, I cut apart the belt and used it for the matching fabric. Make sure you leave enough fabric to overlap the top and bottom hem of your shorts. If you're using a strip of fabric that has raw edges, fold the edges under before sewing your strip on top of the serged edges.
4) Carefully align the front and rear of your shorts before sewing the strip to the font piece.
5) Optional: Serge or zigzag stitch the ends of your long fabric strip.
6) Tuck the ends of your fabric strip into the interior of your shorts and top-stitch to secure the ends.
If you look carefully here, you can see the top-stitching I mentioned in step 6. Along the fabric strip are two rows of stitching. The first was made when the strip was sewn on, the second one secures the ends of the fabric strip to the underside of the shorts.
If the fabric strip was a different color it might even have a nice tuxedo stripe effect.
And that's all there is to it.