Have you tried these? My husband had a bunch of dress shirts that were big and baggy (ahhhhh, the nineties) but soft and in great shape. After a decade of nonuse, however, he was getting ready to toss them. Fortunately I came across this tutorial (somewhere, somewhere -- who led me there? I don't recall) and managed to save a pile of them. The first try got a tie waist rather than an elastic one. My daughter, however wasn't interested in actually wearing it until I finished it off with a horse and her first initial. Funny what a horse and a big L can do for that girl.
Of course, I eventually had to modify, since even general tutorials are too much for me to actually follow. The next two both got the bottom half of the shirt sleeves rather than making a short gathered sleeve and, because I cut it too narrow, the blue one also got a stripe of stripes down the center to loosen it back up. I also skipped the elastic waist. They just didn't seem to need it.
I was thinking it might also be fun to keep the collar on the next one. Any other good dress shirt hack ideas? I'm in love with this one from my cousins over at Sugar City Journal.