Things you need:
- a big, button-down shirt
- 4 or 5 buttons
- needle and thread
- straight pins
- someone to help you, or a dress form
Step 1: Choosing a Shirt
Also, try and find a shirt that has darts in the middle of the back. If the darts are at the shoulders, it won't fit as well. It seems like women's shirts mostly have darts in the middle, and men's shirts have darts at the shoulders.
I used a blue polka-dot shirt for my first dress, and a red plaid one for the second. A Hawaiian shirt could also be a lot of fun.
Step 2: The Structure of the Dress
If you're lazy or desperately need a dress in the next 5 minutes, you can tie the sleeves behind your back and you're finished. However, I recommend cutting the sleeves short and securing them with buttons instead.
You can choose to leave the collar sticking out, like in the first picture, or you can cut it off. It's a matter of how the shirt fits, and personal preference.
Step 3: Cutting Off Extra Bits
Pin the sleeves together in the back and button up the buttons, so that the dress stays up. Then you'll have to fuss around a bit to make sure everything will fold right. You may need to tuck in little bits of fabric along the side, and tug out other bits so they don't get lumpy. These little adjustments don't have to be permanent yet - just make sure that you'll be able to get the front of the dress to look ok, and you can tack down the folds later if you need to.
You'll have to decide whether you want to keep the collar or not. If you don't want it, now's the time to cut it off.
Now you have to cut the sleeves short. Stretch the sleeves behind the person's back, and mark where to cut them. The ends must overlap by at least a few inches, so there's room for buttons and buttonholes and for hemming the ends.
If you started out with a short sleeved shirt, you probably won't have to cut the sleeves any shorter.
Step 4: Hemming the Sleeves
Make sure the sleeves are still long enough to overlap in the back!
Choose some thread that matches your fabric and sew the hem.
Step 5: Buttonholes and Buttons
Pick out some buttons - you'll probably want 4 or 5. Lay them out along the edge of the sleeve, and draw little lines marking the positions and sizes of the buttonholes. Then cut slits along the lines, long enough that you can squeeze the button through.
I hear there's an easy way to sew buttonholes with a sewing machine, but sadly I do not have one, so I'll be doing it by hand. If you're going to do it by hand too, take a few feet of thread, double it over, and start stitching around the edge of the hole. It doesn't have to look pretty (it will be covered by the button) but it should be durable. Try to cover the entire edge with thread, and not leave tufts of fabric sticking out. Take extra care at the ends of the slits, so they don't start ripping farther.
Once the buttonholes are done, it's time to mark where the buttons should be. Have the person put on the dress, and pull the sleeves behind their back. You don't want the dress falling down, so you should pull the sleeves until it's tight but not uncomfortable. If you can't breathe, it's too tight.
Use a pen to draw a mark through each buttonhole, so you know where to attach the button underneath. Take off the dress, and sew the buttons downs at those marks.