Start with your t-shirt, I used an XXL t-shirt, though to big, it provided the length I wanted for a tunic length shirt and with a few well placed gathers, it was flowy enough for a loose fitting summer shirt.
Cut the sleeves off the t-shirt, just outside the seam of the sleeve.
Cut the neck off the shirt, just below the neck hole opening.
Turn under the arm holes and sew so that you have a a finished edge.
Using a basting stitch, sew two rows of stitches on both the front and back of the shirt about 4 inches from each side and 1/4 inch apart. Then pull the bottom threads of each row so that gathers are created. As you can see, I gathered the back of my shirt more than the front of my shirt.
Take your trim and measure out two lengths slightly longer than the top edges of the front and back of your shirt. Fold over the edges of the trim to create a finished edge and sew trim into place.
Measure the length of trim needed to create the straps of your shirt, pin into place and sew.
Yahoo!!! You are done, wasn't that easy? Now to plan what to do with the remaining shirts I bought.