Well, take two units and insert the flap of one unit into the pocket of the other one. Make the two units stand in an right angle to each other. You may want to fix them with a small cloth pin or something like this, but it isn't really necessary. Take a third module and insert its flap into the remaining pocket where the two other modules meet, afterwards put the last remaining flap into the last remaining pocket to finish the first corner of the cube. Go on like this until all the 12 units are in place.