Here is how I turned some old crates into shelves. First off, buy or find some old crates. Usually you can find them pretty cheap in antique stores. I found mine on a garage sale website.
Then I took some tissue paper that I just loved, make sure and buy really big sheets depending on the size of your crates. I cut the paper to fit the boxes and lined the backs. Having tissue paper is nice because it bends and molds without ripping. I just attached it with double-sided tape, but you could use about anything that will adhere it to the wood. They haven't fallen out yet!
My lovely husband put them on the wall. In Germany the walls are pretty much made out of concrete, so you can just drill holes and hang wherever. He put screws through the wood on the top two corners and then into the wall. They are flat-head screws so the paper lining covers them up. You might have to do something different depending on your wall set up. Ask someone handy, I'm not exactly that.
And then decorate! Because of how strong our walls are I had no worries with weight. But still, don't pile your entire set of encyclopedias on there, you don't need them falling off and leaving gouges in your wall.
What do you think?