I'll lay out the steps I used to transform the short-sleeved shirt from slightly stained, drab, and worn out, to fresh, fun, and new again.
Then gather each shirt with narrow accordion pleats and secure each resulting snake-like bundle with rubber bands. Beware: you may experience flashbacks to tie-dye project gone awry, but you should grit your teeth and push forward. This will be way cooler than the tie-dyed shirts you made in summer camp.
Mix up the dye according to the package instructions. I used your basic Rit dye in brown -- you can see it in the measuring cup. It was hard to get all the bits of powder to dissolve, but it doesn't matter as much when you're doing a tie-dye since the outcome will be mottled anyway. The instructions said to wet the shirts and put them in hot water and then add the dye solution, so that's what I'm doing in this shot.
Dye the fabric for the recommended time and then rinse as directed. Here's what the short-sleeved shirt looked like as I removed the rubber bands.
And then as I unfurled the whole shirt.
I then washed and dried the shirt, according to the dye instructions. Then I embellished the neckline of the shirt by sewing on a gathered a section of hem tape that I had in my sewing room. Amazingly, it was the exact color as the dyed shirt! If you really care about matching, you can always dye your embellishment fibers when you dye the shirts. Then I added a decorative running stitch with a contrasting embroidery floss. I wanted another layer of texture and I thought old buttons would be just the ticket. Rather than sew them directly on the shirt and have them be droopy, I sewed them onto a piece of thick felt that I added a pinback to. That way I can also remove the buttons easily for washing. Here's a close-up of the embellishements.