I've done a few of these and I found the easiest way is to use rubber bands to hold the diapers together. You want at least 3 jumbo packages, newborn, 1 and 2. The newborn diapers will be the top 2 layers and for you to fill in any gaps, the size 1 will be your next one or two layers (depending on how big you want it) and the size 3 will be you bottom layer or two (again depending on how wide you make it). Take each diaper and open it up and then roll from one side and ending so that the velcro pattern is what ends up on the outside of the roll (that will help w/ your design), then rubber band it together. Once you rolled them all together. I then take another rubber band and connect it to another rubber band so you have two rubber banded together. Continue until you have the width you want, then fill in the middle w/ the other diapers. Once it is all assembled. Take wide satin ribbon and tie it around the middle of each later to hide the rubber bands. You can then tie accessories around each bundle all around the cake. This is definitely a wow item and I loved doing it! You can get lots of little baby items at the dollar store to tie all over the cake.
If I'm presenting this as a give, I would wrap the whole cake in coordinating tulle (it's make such a nice presentation). If using as a centerpiece, put on top of a cute patterned receiving blanket and save some wrapped diaper rolls (cover the rubber band w/ ribbon) and scatter around the diaper cake. Have fun!
Baby Shower Diaper Centerpiece
Diaper cake
