Step 2: Setting Up Shop
If your base box is a little too big, you can cut it down, just make a straight cut all the way down the long way, bring the two halves together by the right amount, and run a strip of tape down the seam on the outside of the box. It's not load-bearing tape, but it will prevent the box halves from popping apart during the final step.
If your base box is too small... find another box. Or make a base out of yet another of those many pieces of cardboard hanging around.
!! DO NOT ATTEMPT TO ADJUST THE SIZE OF YOUR CAT !!
Take a good look at your big sheets of cardboard, and figure out which way the grain goes. Look at the sides... two sides will have triangles, and two sides will not. You want to position the cardboard with the triangle sides left and right, and the sides without triangles at top and bottom. When you are cutting, you want to cut across the triangles as you pull your blade toward you. If you cut with the grain, it's not as fun to scratch on (or so I'm told).