הלכה על נדה 133:15

And this scrubbing needs to be proximate to her immersion - meaning to say that she not be involved in anything in between. And nonetheless if the immersion of a woman happened to be on Shabbat or a holiday, they permitted even from the outset that she scrub during the day and immerse at night. And likewise the Sages, may their memory be blessed, said (Niddah 67a) that on account of the concern that water should enter onto all parts of here body, a woman needs to stand at the time of the immersion in such a way that she appear like a weaver or one nursing her child standing under her breast. And she also needs to be careful about rings on her hand and bracelets - and if they are tight, she must take them off her hands at the time of immersion; and so [too,] to untie the bands on her hair, and to rinse her mouth and teeth. As even though water does not need to come into any of the hidden places, they need be fit for the water to come in. And there was a case (Niddah 66b) of a woman who immersed and a bone was found between her teeth, and the Sages required her another immersion. If she immersed and then something else separating was found upon her, such as dough or tar or dry blood and what is similar to them from all of the things that the Sages enumerated as separating - even it if is as small as a mustard seed, so long as she is concerned about it and her way is to remove it - her immersion did not count for her. And even if the thing was under the fingernail, and even if it was [impressed on] the flesh - since she is concerned - it is [considered to be] separating. And therefore the daughters of Israel have been accustomed to cut their nails at the time of immersion. And the rest of its many details of the laws of separations and the laws of mikvaot and one who has immersed on that day are elucidated in Tractate Mikvaot and and in Tractate Tevul Yom.