I made this out of a thrift store bra. It was a strapless bra so the top side was relatively straight across. I used a seam ripper to separate the cups from the rest of the bra, and I took the underwire out of one side.
I carefully matched the cups, then put a zipper on the upper edge. I then sewed the two cups together with a straight stitch along the edge. I then free-hand embroidered the cat face with fleece scraps and old buttons, then attached a rick-rack strap. I made it up as I went along, so I wanted to sew first, but this is not very efficient - if you have a clear vision of what you want, embroider first.
The picture makes the bra look a little glossy, but I took a poll among friends and no one could immediately identify it as a bra. I am giving it to a little girl so I didn't want it to be too obvious!