Sharpen one end of a dowel with a pencil sharpener, then pound it through the boards with a hammer. It sounds violent but it works. Make sure to cut the dowel to the correct length first, or you'll have to do it while it's sticking into the cake! 
You use 1/4" foam core board or cardboard (covered) and you sharpen the end of your dowel like a pencil, then take a small hammer and hammer it through all layers into the board. (I use 1/2" foamcore board as my base so the dowels go through it easily)
Sharpen one end of a dowel with a pencil sharpener, then pound it through the boards with a hammer. It sounds violent but it works. Make sure to cut the dowel to the correct length first, or you'll have to do it while it's sticking into the cake!
Second the motion!! 
I tried the hammer thing, didn't work. Here's how I did mine and it is just preferance and as soon as I'm good enough and have more practice, I'm sure I will be able to "hammer" the dowels in without destroying the cake! I started with the top layer (which I did not have on a plastic cake plate but a cardboard circle, I pushed a "skewer" (works better than a dowel that you have to sharpen ) through the cake and cake board. Took it out and did it two more times. Remember that you're skewer/dowel needs to be cut to just the right size!!!!!!! Put the top back on and then push the skewers back down through the top layer (should slide right through) down through the second layer. I then used a dowel to "TAP" the skewers into the board. 
Hope this helps.
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