Step 1: Materials and stuff
Sticky tape (Not much was actually needed)
Cardboard boxes (Same size helps)
You don't really need all of these items as it's not going to blow up if you make it a different way. You might as well experiment a bit!
Step 2: Start planning and cutting
So, grab the first box and start out with cutting a hole in the side where your cat can enter it by just walking in. You can leave a flap for a door or just cut it right out. Fold the bottom flaps of the box and cut the top flaps off as seen in the photo.
Start cutting out some rafters to support the second story. Mine went from one side of the box to the other and protruded further by about an inch or so. Make sure the rafters areat least a few inches thick to support the weight of the cat sitting above them on the next story. The rafters are linked together so that they support eachother and share the weight. To link them like this in the photo, you will need to make a cut half way into each rafter so that you can slide them together. Make sure to measure the cuts so that you don't bend the rafter too much - keep them nice and straight.
Just study the photo to get the idea...