The leftover scraps became a fun, braided headband that matches the top perfectly.
Step 1: Cut Your Shirts Into Strips
Select two coordinating t-shirts and cut off the sleeves, hem, and collar. Make lengthwise (hem to shoulder) strips out of the front and back of both t-shirts.
I made strips that were 3 inches wide and 19 inches long. I needed 18 strips to make the top as long as I wanted it, but you can certainly keep adding and adding to make a dress! You could also cut wider or narrower strips to suit your taste, or even use three or four different shirts.
Be sure to save those scraps from the sleeves; you'll need them in a few minutes.
Step 2: Sew the Strips Together
Make sure that you're placing the right sides of the strips together. If you aren't sure which side is the right side, gently tug on the strip. The fabric will curl a little bit towards the wrong side.
Now take the other half of the strips and sew them together too, but offset them in the other direction. If the first panel leans to the left, you need the second one to lean to the right. So, if you were letting the extra inch hang off the left side of the strips when you sewed the first panel, let it hang off the right-hand side when you sew the second panel.
The seam allowances should curl open by themselves, if you have the fabric oriented correctly.
Step 3: Start Assembling Your Shirt!
Now try the top on while it's still inside-out, and use safety pins to pin the two sides together. Start below each arm, and pin it all the way down to the hem. Make it fit the way you want it to, but leave extra room to get in and out of it. It's always better to start off with the top fitting you too loosely, since you can take the seams in later, but you aren't going to have much luck putting the fabric back after you trim it off.
When you're satisfied with the way it fits, take it off and sew the side seams where the safety pins were, removing the pins as you come to them. Remember to keep that opening below the shoulder seam, so that your arm can go through! Now, trim away all the excess fabric that's left beyond the seams, and trim the armhole to make it neat and tidy.
Step 4: Add a Second Strap
Try the shirt on, and drape a sewing tape measure over your bare shoulder to determine how long your strap needs to be. Mine was about 9 inches long.
I also added trim to the edges of my strap. You can use lace, or elastic, or just leave it plain if you want to. I was lucky, since the flowered shirt that I cut up had trim around the neck when I got it. I removed the original trim with a seam ripper, and placed it on the right side of the strap. I aligned the flat edge of the trim with the cut edge of the strap, and sewed right along the bottom of the scallops. Then, I flipped the trim to the wrong side, so that only the scallops peeked out from underneath the shirt. I topstitched the trim to hold it in place. (I used the same process to trim the other armhole, too.)
Try the shirt on, and use some safety pins to try out different places for the strap. If it's too far to one side, it may rub your neck. If it's too far to the other side, it might fall off your shoulder! Take your time and figure out the very best place for it. Hold it there with the safety pins, then take the shirt off and sew the strap to the shirt.
Step 5: Final Fitting; Finish the Edges
You can simply leave the edges raw, if you like! They won't unravel. If you'd like to finish them, you can fold them under and topstitch them down, like I've done here. You could also sew elastic to the edges, or lace, or any kind of trim that you want. You can braid leftover scraps of fabric together and use that for a matching trim. The possibilities here are limited only by your imagination and whatever you have on hand.
Step 6: Make a Matching Headband
Measure all the way around your head, just behind your ears. My measurement was 22 inches. Now measure the braid and cut it to 5 inches shorter than your head measurement (17 inches, for me.) Sew the other end of the braid together to keep it from coming undone. Cut a 5-inch long piece of elastic. Overlap one end of the braid and one end of the elastic by about a half inch, and sew over them a few times to make a sturdy seam. Repeat with the other end of the braid and then the other end of the elastic, to make a circle. Make sure that the braid and elastic aren't twisted, before you sew!