I had to do some tests to figure out how wide to make my slots. Using the same material, I cut out a bunch of rectangles with differently-sized slots and saw how they fit. I settled on slots 0.195" wide for the 3/16" plywood I was using. They were snug, but didn't require a hammer to push together. In retrospect, I should have made them looser, with "nodes" to help the slots bite. You can see an image from Ponoko's manual
for this (it's the second one.) That would have allowed easy assembly without too much forcing.
Okay, once I knew what kind of slots I was using, I made some little slot templates to make adding them to the spines and cross-sections easier. The idea was to measure the place where the slots met using the top-most curve of each shape. You can click through the two series of images, below to see how it was done. The image notes have all the details.