The porticos tabs fit into the proscenium slots.
The portico attached to the proscenium.

Attach the portico to the proscenium.

Cut out the holes (marked in red in the image at left) on the proscenium that will receive the tabs of the portico sides.

You will have scored the tabs, and after inserting them into the proscenium slots, apply glue and fold toward the stage.

Place creche flat on a flat surface, portico side down, and press down on each tab insuring a good bond.

Repeat for other side.