A blunt screwdriver works well to score the interior lines. There's also a tool on most pen knives that is great for this
Crease the lines
Fold all the interior lines into mountain folds, except for the "double tabs".
Attach & Glue the band
Put the rubber band through the double tabs, and glue it in place. Although these photos show otherwise, the band should be attached in a figure of eight, crossing in the middle. At this point, the rubber band will start trying to destroy the cube, so be careful!
Glue the remaining tabs
The strangely shaped tabs at the side need gluing in pairs. Once you've done this, apply glue to the final tab, and carefully allow the cube to close.
You can now flatten your cube, and deliver it in an envelope, or slide it under someone's door.