I am going to be making one soon, so I mapped it out on paper last night. I'm probably going to do 4x6x4 infant prefolds (washed and ready to use). I measured a rolled one last night and decided I would need 24 for a two tier cake.
Here's what you do: Fold a prefold in thirds, roll it tight, and tie it with a ribbon. Do that to all of them. Place them standing up on a 10 inch cake board. One should be in the middle. There should be six in a circle around that and 10 more around those. The top tier would just be the middle pf and the circle of 6. Everything should be tied with ribbon. Once you have it all together, you can tie on extras (baby bath stuff, snappis, pins, ect).
I was also thinking of doing prefolds for the bottom tier and wipes for the top tier.
Have fun with it!