Lie shirt flat on a surface.
Cut sleeves off, as close to the seam as possible.
Draw a straight line across the chest, from one bottom of an armhole to the other. Cut along this line, through both layers of fabric.

Create two knife pleats on each side of the front buttons, keeping them evenly distributed. Baste across the top, raw edge of the shirt to hold the pleats in place.
Topstitch the pleats a few inches down from the top edge, if you'd like.

(Another view of front pleats. Sorry it's hard to see with the busy fabric.)

Create another pleat in the center back. This time it's a box pleat/inverted box pleat.
Baste and topstitch as you did with the front pleats.

Measure your chest, just below your underarms (where you would wear a strapless top).
Cut elastic an inch or two shorter than this measurement.

Create a casing for the elastic along the top, raw edge, leaving an inch or so open.

Thread elastic through the casing using a safety pin.
Stitch ends of elastic together, making sure it's not twisted.
Stitch casing opening shut.

Adjust elastic so that the fabric is evenly gathered. I stitched the elastic down perpendicularly on the side seams to prevent twisting (totally optional).

Finally, cut three long strips of leftover fabric from the shirt (or other random scraps).
Knot and braid them, then tie around the waist.
Voila! You now have a simple strapless top that will keep you cool on even the hottest summer days :)

(This top can also be worn as a skirt!)