vidhinā vidhi-kovidaiḥ

trayodasyam-on the thirteenth day of the moon; atho-thereafter; visnoh-of Lord Visnu; snapanam-bathing; pancakaih-by pancamrta, five substances; vibhoh-the Lord; karayet-one should execute; sastra-drstena-enjoined in the scripture; vidhina-under regulative principles; vidhi-kovidaih-assisted by the priests who know the regulative principles.