I unfortunately didn't manage to get a picture of Clara actually playing with these blocks, however I did happen to have a couple of milk cartons ready to go so I can demonstrate this complex procedure! ;)
First, collect as many 2L milk cartons as you can get your hands on. They need to be cartons, not jugs. We put the notice out to our family members and switched to using cartons exclusively for awhile, and they started pouring in! Each block uses two.
Cut the tops off of both milk cartons. Depending on how soon you cut the tops off after each carton is used, it's easier to clean them after they are cut. If you clean them immediately, just a few quick rinses seems to do the job. Avoid waterlogging them, though - the outside and cut edges are not as liquid proof as the inside!
Make sure the cartons are completely dry, and then slide one carton inside the other until they are as tight as they can go. That's it!
These are a little cumbersome to store, but trust me when I say these are awesome toys. I remember my preschool having about a zillion of these - we could build forts that were taller than we were, and we even managed to build windows and doors into them. I want to continue to collect these over the next couple of years until Clara is old enough to really appreciate them, and to build forts! Now she just waits until I build a tower and knocks it down, followed by some evil laughter... she'll get there.