Fold the diapers in half, and alternate the way you stack them. I believe I used 15-17 for each row on the bottom layer, but you can make yours smaller or larger. If you're going to stack more than 15 or so, I suggest using rubberbands to secure smaller stacks, and then wrapping the ribbon around the entire row.
I cut the ribbon to the length I needed, and wrapped each row with a thick piece of satin, followed by a very thin piece of satin. You can use any type of ribbon, or even tulle – whatever you have on hand is best. I secured the ribbon with a small piece of rolled up tape in the back of the cake. NONE of the tape was visible on this diaper cake, but I used a LOT.
After wrapping vertically with ribbon, wrap it horizontally as well. It really makes it “POP”. Use scrap pieces of ribbon to make little bows.
Add toys, baby wash, etc to your diaper cake – secure with hidden tape if possible. With the round diaper cakes, I always put a baby bottle, baby shampoo, or a big box of wipes in the center – so that's a possibility as well if you are trying to put as many things as you can in/on your diaper cake.