Okay, I have to be honest. We didn't eat this one. But it was lots of fun to make for my coworker's baby shower, and pretty easy.
I purchased a wooden dowel (about 12 inches) and wrapped a couple of baby blankets around that securing it with rubber bands. Then I started layering diapers around the bottom (size 1's) until I got the base as wide as I wanted. I used dental floss to tie them together. After that I repeated the process with the top layer and decorated it all willy nilly.
I was hoping people would start hiring me to make diaper cakes on a regular basis so I could quit my day job, but so far no luck.