First, cut out the main wall of the building. Note that the three corners of the building are not sharp right angles but instead are rounded. Carefully bend the corners over a stick or small dowel to give them a smooth round curve. Glue the end of the wall to the flap to make a triangular shape.
Cut out the larger triangular piece and glue it inside the main wall. The top of the triangular piece should be flush with the top of the wall or even slightly inset from the top of the wall.
Then glue the triangular rooftop with rounded corners to the top of the building.
Cut out the railing and curl its corners in the same way as the main wall of the building. Glue this to the rooftop so that it lines up with the walls beneath the roof.
Cut out and assemble the triangular mechanical level of the building. Glue this in place on top of the roof
Cut out the tiny glass showroom. Again, bend the corner to a small curve instead of a creased point. Glue the sides of the base of the front corner of the wall of the building. Then add the tiny triangular roof atop the showroom.
And now your model of the Flatiron Building is finished!