This skirt was almost exactly one pleat's-worth too wide. To alter I basted where the new side seam would be. Then folded out the pleat and pinned the new side seam to check fit. It was pretty good so I unpicked the side seam and right side of zipper, and sections of the waistband and hem. Double checked the fit, and cut off the extra fabric.
I resewed the side seam first, pinning to match the plaids. The zipper is topstitched on so I basted it into place, matching the seams, and stitched down the base of the left side, across the zip, and back up the right side. The left side has the layers (pleat and pleat underlap) edgestitched together then topstitched to the zipper at 1/4". The pleats are hand stitched together just below the zip.
I used pinking shears to remove the excess fabric. The original seams had been pinked and are holding up well. The hem is covered with seam binding and was machine blind hemmed. I sewed the seam binding back on, removing a damaged section. Then steamed the hem to an even curve and whip stitched the hem back down.
Someone had done alterations on this skirt already. The waistband had a length extension. The zipper had a couple lines of stitching and some unpicked, but not completely removed, thread. I'm not sure whether someone tried to alter this to be wider, or narrower, or ? I didn't see any evidence of alterations in the side seams or pleats. Maybe she just added width to the waist as girdles fell out of favor. The etsy seller I bought this from had a matching jacket, maybe it provided the extra waistband fabric.
I removed most of the waistband extension before edgestitching the waistband back onto the skirt. The raw edges of the underlap were turned in and whipped together to minimize bulk. It looks funny but works great. The button overlap now covers the waistband extension.
Most of the shaping in this skirt is from angled pleats. They're narrower at the top, taper out to the hip, where the pleat topstitching ends, then are free to the hem. It's a lovely skirt, lightly worn with no holes, and a nice just-below-knee length. It requires control top pantyhose or a girdle since the waistband sits around my natural waist and the pleats add bulk, accentuating any tummy. I don't mind wearing vintage undergarments with my vintage clothes, though. At least not in winter ;)