i picked up a floral print blouse at Salvation Army for a dollar (r.i.p. dollar days). the blouse was too large so i used the fabric for the shorts, which are vintage Levi's from ebay.
this diy is so simple. i suggest taking a sheet of paper and laying it on top of the shorts and tracing out the shape you want with a pencil. then, cut the fabric according to this stencil and attach to the shorts with fabric glue, liquid stich, or sewing it on.
as you can see, i attached a piece on the front and the back, on opposite sides. this adds some extra visual interest to the shorts. good luck!