They connect together with a small peg and hole and a snap on the sides.
To hang, I simply drilled a hole in the bottom of each crate, used an anchor and screw to attach to the wall and Viola! I would also recommend using a large flat metal washer behind the screw, since the plastic can crack, it will also add stability.
The hardest part was getting the stickers off the back. ;)
***For those that have expressed concern about the height of the crates, my house is a ranch and the upstairs ceilings are low. Two walls are slanted, so wall space is limited, I had to be creative. My daughter is almost 8 and is tall, so with the help of a small step stool, she can reach her toys. If your child is smaller, I would recommend hanging these crates lower.