The assembling step is a step of preparing an assembly. The setting step is a step of setting a plurality of holder installation areas along an outer peripheral edge of the peripheral wall. The determining step is a step of determining a first area in which the slit and the peripheral wall are not adjacent to each other in the first area and a second area in which the slit and the peripheral wall are adjacent to each other. The installation step is a step of providing a holder in the first area without providing the holder in the second area.