Here's what you'll need to build your cake:
...a few cardboard boxes that are in good shape, a pencil, scissors, and most importantly, paper packing tape. You can get a roll at an office supply store like Staples. My dad gave me this roll, believe it or not, about twenty years ago when I was making paper mache animals for the preschool where I worked. It was much bigger then, but it's like the ever-lasting gobstopper of tape, the only catch is that you need to store it in a dry place. If you don't, the humidity will cause the glue-coated side to stick the roll together and it will be useless. This tape is important because any kind of plastic tape will not allow paint to stick to it. You don't want to do all this work and then be unable to decorate your cake!
Now, take one of your boxes and round out the sides, like this:
The more bends you put in the cardboard, the rounder your cake layer will be.
Next, lay your carboard box back down on a flat surface, take a straight edge (I use a 48" long metal ruler I bought in the Sears tool department for this kind of thing) and draw a crooked line across the top. The more crooked, the better. You don't want people thinking your cake layers are crooked by accident. When I brought my cake in to school for the play, I was informed by one of the technology teachers that I'd created a truncated cylinder. "Yeah, yeah, that sounds familiar," I said. Don't ask me to find the volume of a truncated cylinder. That's about the time my math anxiety kicked in.
Now you're going to put a bottom and top layer on your cake. Place the your cake onto a piece of carboard that's large enough to cover the whole circle you've created. Arrange the bent cardboard cylinder into a nice circle shape, unless you want a truncated oval, but hey, you're on your own with that. Trace around your cake, drawing a circle on the flat piece of cardboard, like so:
Cut the circle out with your scissors, or a box cutter if you like living on the edge. Place the "cake" on it's bottom. Does it fit pretty nicely?
If so, flip it over and tape around the edges with SMALL pieces of your paper tape. You need to use small pieces of tape so you can keep everything totally flat while navigating the edge of the circle. If you use big pieces, they won't lay flat enough. The tape I use needs to be wet on the glue side to activate the glue (hence, the humidity problem). There may be other tapes that work, but I haven't used them, so I don't know for sure.
(In this picture there's no bottom yet, but I added it later. I should have added it first.)
For the top layer of your cake, repeat the process described above, except use a smaller box for your initial circle (my 3rd picture). OR, you could do what I did, and cut a chunk out of the box, and use your paper tape to tape it back together. For this second layer you don't need a bottom, just a top. When you attach it to the first layer, you may have to spend some time trimming the bottom of the top layer so it fits on the bottom layer as snuggly as possible.
Before you decorate, make sure EVERY hole is covered up with a smooth layer of paper tape. Holes will show up glaringly when you decorate.
A friend of mine is going to make an UNtruncated (regular) cylinder cake for a shower she's planning - she's going to use it as a platform for her TERRIFIC cupcakes. If she starts a cupcake blog as she says she's going to, I'll post a link here.
Have fun everyone, and Merry Unbirthday!