That small piece that was left over from the first piece of siding will be attached to the left side of the garage door on the front of the diorama building. You have to pay particular attention to the positioning of this piece it has to line up with the lines from the side piece of siding.
You also have to have it in the right place where it meets the garage door opening. There will be a piece of trim work that goes around the door opening so use a small piece of trim to line it up so you don’t have a gap between the siding and the trim piece.
The outside corner remember will be covered just make sure that you don’t have too big of a gap but a small gap will not hurt anything and will not be seen when the corners are finished off. Glue this piece to the front & use clear tape to temporarily hold it in place until it dries.