You should do your preparations thoroughly on erev Shabbat. You may do some additional preparations on Sunday night after nightfall prior to your immersion. You should not take a bath on Yom Tov, but you may shower in warm water if you follow the position that it is permissible on Yom Tov. Please see our article on
Chafifah on Yom Tov for more details and guidelines.
Chag Sameach!