After unpacking the glasses, fold the empty packed again. The littles holes go below the side with the notches, by pressing them into the holes you'll fixate the carton as a cube again. Use tape to fixate it even better.
Step 2: Connect Two As A Base
To have a base for the second an third level, connect to of the cartons to one. The third one will lay on it.
Step 3: [Optional] Connect Two Staircases
To have a wider rack, you can connect the cartons by gluing them with a peace of tape.