I cut them apart, sewed them into skinny shorts because I love the brightly colored skinny pant look right now, and after several dyeing misadventures I had these (still wrinkled from drying the dye).
I constructed the whole shirt out of solid fabric, but didn’t sew up the side seams. Then I drew a paper template for the waves and traced it onto the shirt front.
Then I cut the t-shirt, setting the bottom part aside. I laid a very wide strip of the next color (almost not wide enough) under the cut edge and pinned. Then I sewed right along the shaped edge, kind of like reverse appliqueing.
Cut the next edge and repeat with another color – not pictured, but it’s the royal blue in the finished picture.
Finally, pin on the bottom of the shirt you cut off in the first step and sew it on. Then sew down the side seams of the t-shirt and cut the excess off the bottom front. Hem as usual, or leave unhemmed.