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...
Step 3: Stair case!
I cut out two triangles of the same shape and size with a right angle so the stair case can sit against the box wall. These two triangles are each side of the stair case. Seven or eight stairs will get your cat up there but that depends on the size of the boxes.
You need a bit of patience with making the stairs as it can drag on a bit. They came out surprisingly sturdy though so give them a go. Just make sure to study the photo to get the idea of what it will need to come out like.
Step 4: First story complete
Step 5: Second story
Using that thrid box, cut out the flooring for the second story and the perch/balcony. You only need enough tape to stop it from moving. The support for the floor is in those rafters!
Grab that second box and put it on the top. Use a few bits of tape to hold it in place on there. I cut another doorway for the cat to get out onto the perch/balcony. The cat will probably prefer to jump up onto it once it's done so make sure it holds the weight if you decide to make one.