When constructing a diaper cake, how do you get the bottom tier to attach to the circle base?

You're not dum by any means!!!!! You don't need the dowels to secure the tiers. I've done that and there is no difference between having them and not having them! The diapers are going to shift somewhat. Just be sure to put enough diapers and secure it tightly with ribbon. I discovered that using clear elastics (found at the hair accessories department at any Target or Walmart) works best on keeping the diapers wrapped individually instead of tape. And what I use is as the base to hold the diaper cake, is a cake platter with ruffle skirt already attached to it. I get that at AC Moore. It comes in different sizes and price ranges from $2.99 for small to $5.99 for the xlarge. To complete the look, I buy tulle to match the color scheme and wrap the entire diaper cake. That will definitely keep things from moving around, and it looks amazing. I can email you a picture of the one I did: unforgettablemomentsbyandrea@yahoo.com. Good luck!