You must complete all preparations on erev yom tov (Wednesday), and try to avoid chatzitzot throughout yom tov to the extent possible. Before immersion you should perform all preparations which are permitted on yom tov: brush and floss your teeth, wash your hands, feet, or other parts of your body individually with liquid soap (bathing the entire body is not permitted), and untangle any knots in your hair with your hand (not with a brush or comb, which might lead to the prohibited act of pulling out hair). Most importantly, you need do a thorough iyun (inspection of your whole body in front of a mirror), immediately before immersion. We suggest you double check that you have cut all your nails before chag, since you cannot remove them on yom tov.

Please read our article Chafifah on Yom Tov for a more detailed discussion of mikveh preparation in these circumstances.

Behatzlachah and Pesach Sameach!