Cut out the IDS tower. Score and fold into an octagonal shape to match the roof. Glue the walls together and glue the roof in place.
Score and cut out the base of the IDS Center. Glue the walls of the base square. Note the small reverse fold for the walls near the base of the tower.
Glue the tower to the base, with the tabs on the tower matched up to the place indicated on the base. The octagonal end of the base fits into the bottom of the tower.