Gather these parts

Slot the sides into the back plates with glue as shown

Glue detail plate on with the top even with the top edge of the back plate

Glue the lower platform onto the previous steps slotting the pegs into the holes

Glue the top platform onto the previous steps slotting the pegs into the holes

Glue the support ribs into the slots from underneath so that the ribbed texture shows on top