What you'll need:
A button-down shirt that's a couple sizes too big
Stretchy fabric in a contrasting or matching color
Sewing machine/needle and thread
First, try on the shirt and decide where you want to put in your waistband. I decided to put my waistband right under my bust because that's the smallest part of my torso. Cut the top of your shirt about an inch below where you want your waistband to go.
Now measure around your waist where your waistband will fall. Divide this measurement in two. Take your stretchy fabric and fold it in half. From the folded edge, measure out half of your waist measurement. Cut the band twice as wide as you want the final band to be. Make sure you add at least an inch to both the length and width for seam allowance.
Fold your band in half so it is the width you want it to be. Pin it to your shirt's top, right sides together, pleating the shirt as you go to make it fit the band. I did this randomly, but you may want to make your pleats every few inches so it looks even.
To help me, I pin the middle of the shirt to the middle of the band, and the sides of the shirt to the sides of the band and then pleat.
Sew the band to the shirt.
It should look like this when you turn it right side out.
Now pin the bottom of your shirt to the bottom of the band, right sides together, and sew.
Line up your shoulder seams and cut the sleeves about 2 inches longer than your desired length.
Now, try on your shirt again and decide how low you want the scoop neckline. I wanted mine to be off the shoulder without being too low. Cut the neckline, making sure to make it a bit higher in the back.
Don't worry if your neckline isn't even. It's going to be scrunchy and off the shoulder, so it won't look uneven when worn.
Now we're going to make a casing for the neckline's elastic. Fold the cut edge over about an inch and then fold it over another inch, so the cut edge is inside the casing. This will prevent fraying.
Sew your casing as close to the edge as you can. Make sure to leave a hole to feed the elastic through. I wanted to be able to button and unbutton the shirt, so I left the sides of the shirt where the buttons are open.
Put a safety pin on the end of your elastic and feed it through the casing. If you're feeding it through the button placket of the shirt, like I am, make sure you don't let the other end of the elastic get pulled through the hole on its side of the placket.
Sew the end of your elastic in place by sewing through both it and the button placket using a zigzag stitch. Do this on both sides.
Wrap some elastic around your arm where your sleeve will fall and cut it. Sew a casing for your elastic and feed it through and sew just like you did for the neckline.
You can leave your dress like this or you can do what I did and add straps. I just used the collar of the shirt to make my straps and sewed them down right under the elastic.
There you go--a fun, girly dress! Mine turned out a bit short, so I'm wearing it with black shorts and leggings.
I hope you enjoyed this tutorial, and, as always, feel free to leave any questions, comments or suggestions below!