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.