the third picture you can see where i marked a straight line from the shoulder down to the waist, all but removing the sleeves and armscyes. i marked this after aligning and pinning the bottom hem, and pulling the shoulder seam taut. and you can see my super high tech sidewalk chalk. no expensive sewing chalk for me. serge along the chalk line and open shirt back up, cutting open shoulder seams if necessary to lay flat.
a more experienced wiser sewist would iron stuff at this point. or well prior. i dont iron if at all possible.
i basically matched up the front and back [folded over the gathered skirt portion] and sewed a line right over the previously sewn back seam. i also sewed a quick seam across the top so the waistband would lay flat since lazy me didnt use interfacing.
if you left enough room at the ends in previous steps, it should be easy to tuck in serged edges, pin the ties, and sew shut. i unpicked a few stitches and of course since i backstitch by sheer habit, those few stitches were slightly north of a goddamn hassle.
*edit - i made another after posting this. it took me an hour. and not just cos it only has one pocket. since i basically mucked about figuring out what i was doing [and pausing for pics] it took me much less time the second attempt. learning curve and whatnot. but another unworn thrifted shirt put to good use!