I have stewed about this for a while and thought of several solutions. This is the one I decided to go with because it was quick, easy (of course), and cheap. It will be perfect for spring in Michigan since spring is often pretty cool here. Instead of a shirt, it will be a dress.
You can easily do this, too, with any shirt whether it has stains or not. All you need to do it cut off the shirt and add your skirt part. You could lengthen the life of a shirt this way, too, and just make it a longer shirt. The directions are the same for a skirt or a tunic-style top; just change your measurements. You can also had a ruffle or other decorative element. It is pretty versatile. I kept it simple since my fabric is pretty busy and the shirt is already decorated with colorful buttons.
Cut a rectangle to make the "skirt" part of the dress. I actually cut mine the "wrong" direction because I wanted the stripes to be vertical not horizontal. Since I am gathering it, this shouldn't be an issue. I almost doubled the width I needed so I would have plenty of gathers. I used another dress to determine the length and came up with 18 inches which included my seam allowances.
Next sew the gathering stitches at the top (width) of your skirt part. You need to sew at least two (3 works wonderfully better) rows of your gathering stitch. It is simple, but it makes a huge difference in how smoothly your gathers fall when you sew it to the shirt. Start your stitch about 1 inch in from the edge and end about 1 inch from the opposite side. I sew at 5/8 of an inch, which is standard, and then again about 1/8 (or more--there is no exact science to this--just fit in two more rows before you sew off the edge) of an inch closer to the edge and then another one another 1/8 inch closer to the edge. It is a little tedious, but it goes fast and, trust me, it is worth it. If you set the tension to the highest number and your stitch length to the longest stitch, your machine will do most of the work for you.
I like to start the hem next, but do not sew it down yet. You want your hem to be sewn last so the side seam gets tucked into it for a more professional look. You can go here (look for the turquoise highlighted part at the bottom--it was a long post!) for a detailed explanation on doing the hem first minus the sewing it down part.
Next, open out the ironed down hem and sew/serge the side of your skirt together.
Cut your shirt off being careful to leave a seam allowance if you need to.
I wasn't sure how to do mine since it had a ruffle attached to it and a seam in my way. I didn't want to cut above that point because it would be too short and I worried it would get bunchy under the arms. I was too lazy to take out the top stitching and the stitching to remove the ruffle. In the end, I cut off the ruffles as carefully as I could so the seam allowance was as even as I could make it without actually meausuring. I left the previous seam in but had to sew it down again later when I topstitched. And, this also meant that my new seam had to face downward instead of upward. A little weird, but it seems to have worked. None of this is an issue if you don't have a seam at this point on your shirt. This is what mine looked like.
Turn the skirt inside out and put it around your shirt so the bottom edge of the shirt and the top of the skirt are lined up. (Your shirt will be right side out inside of your skirt. This puts your right sides together.)
Pin the sides with the seam and marker first. They should line up with the side seams of your shirt. If your shirt didn't have two side seams, you should find the sides and mark them just like you did on your skirt piece. By lining these up first, you are helping to distribute the gathers evenly around your new dress or shirt.
Gather up each skirt section until it matches the width of the shirt in the front and the back. Pin in several places to hold the skirt section. I pinned a lot because I wanted to be certain I was sewing where I wanted to--it was a little crazy with that extra seam in the way. In hindsight, I should have just taken it out along with the topstitching.
Sew in place using a regular stitch. I used a 5/8 inch seam. After it is sewn, turn everything right side out and check to make sure it is right--it is a lot easier to fix it now if you need to do so. If it looks good, serge the edges if you would like. Clip any wayward threads.
On the outside (right side) iron the seam smooth while making sure the serged edge underneath is facing the top of the shirt.
I added a quick flower embellishment because it was easy. Actually, so easy that I made two so I could use one for a hair clip. Take a strip of your leftover shirt piece (I cut mine down to about 1 1/4 inches wide after the picture was taken) and run one gathering stitch along the long edge. Mine was pretty close to the edge--maybe 1/4 inch.
Make a flat circle with the gathered strip. Then, while holding the first one, make another one behind it and another one behind that. My strip ran out at that point. The circles won't lay perfectly flat, but you will beat them into submission when you sew them down. I just sewed around the center section kind of randomly so that it sewed through all of the thicknesses. Sew on a contrasting button right in the center of your flower.
Since I didn't know exactly where I wanted to place my flower on the dress, I just put a pin through the back so I can try it in a couple of places before I attach it by hand. I also made another one into a hair clip. This one can be hot glued to whatever kind of clip you want to use. Make two more and you have shoe decorations.
Oh, the possiblities are endless.
Just don't wear them all at the same time!