To prepare it for mosaic, I first vandalized it by scoring the surfaces I was going to stick the tesserae (that's the fancy word for little tiles for mosaic; another fancy phrase I learned is pique asiette, which means mosaics made from items like broken dishes) to and then I painted it a nice, bright tangerine color with several coats of chalk paint. Then the fun began.
At my friend's mosaic gathering, I stuck shards (most of which I'd previously broken up) into mortar on the back panel of the shelf, with a butter dish finial and teacup handles for hooks. This mortaring part is what had held me back from trying mosaic on my own all these years, but it turned out to be incredibly easy. A lesson there.
And now I just need to figure out where to hang it up--and what to mosaic next!