The video link Prism99 posted shows quite clearly how to put the nappy cake together. It's up to you what you actually use in it.
I used nappies on the bottom layer, bibs and growsuits in the second layer and singlets in the top layer. I used a bunny rug to wrap around the outside of the layers without wrapping a nappy around first.
The bibs, growsuits and singlets were rolled the same as the nappies, just making sure they were even in height.
My daughter (who is also expecting later this year) is making a bouquet using baby socks.