You basically arrange the rocks into a spiral formation, fill in the rest with dirt, and select your plants. It seems best to put the shorter ones on the bottom and taller ones on the top.
My plan is to put one of these in our front shady yard, where we have a lot of space, but the ground is pretty difficult to dig up due to the large amount of trees and their roots. Since you build it off of the ground, you don't need to dig things up. Due to the spacing, you can also have things grow close together and fill in the gaps. I think this could also be really awesome if you had a lot of sun, and you could pack it with succulents!
Tomorrow, I'm off to City People's, where they have the most amazing collection of plants I've ever witnessed. Until recently, I'd only been to the inside of the store- and while I always found things I wanted to buy, I was not as impressed with their plant selection. After a visit to the back nursery, I was completely blown away. They arrange plants by type (annual/perennial) and shade/sun. There's also a section of entirely native plants! They also label ones that are liked by hummingbirds/bees/butterflies/etc. I'll be sure to take some pictures to share!