what you might not learned with your Kallah teacher. part 1

...She must dunk her whole body at one time; therefore, it is necessary that there not be on her anything that is a barrier. And even if it is a miniscule amount, if [this thing on her is something] that people are sometimes concerned with [i.e. they would try to remove it], it is a barrier, even if she is not currently concerned with it, or even if she will never be concerned with it, since it is the way of the majority of humanity to be concerned with it, it is a barrier. And if it covers the majority of the body, even though humanity is not concerned with it, it is a barrier. {Rema: Ideally, she should not dunk even with things that are not barriers, which is a decree because of things that are barriers.

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When either a human or a k'li is immersed, there should not be any intervening substance between them and the water. If there is an any intervening substance between them and the water - for example, there was dough or mud clinging to the flesh of a person or the substance of a k'li - the person or the k'li is impure as it was originally and the immersion is invaild.

According to Scriptural Law, if there is an intervening substance covering the larger portion of the person's body or the larger portion of the k'li, the immersion is invalid, provided he objects to the intervening substance and would like to remove it. If, however, he does not object to it and does not pay attention whether it would be removed or not, it is not considered an intervening substance, even if it covers the major portion of the person or the k'li. Similarly, if it covers less than half a person's body, it is not considered as an intervening substance even if he objects to it. According to Rabbinic decree, any intervening substance that one objects to invalidates an immersion even if it covers only the lesser portion of the person's body. This is a decree, lest such a substance cover the greater portion of the person's body. And any intervening substance that covers the larger portion of the person's body disqualifies the immersion even if he does not object to it. This is a decree, lest a substance that one objects to cover the greater portion of his body.

Thus if even a drop the size of a mustard seed of an intervening substance, e.g., dough, tar, or the like, was on a person's flesh or on the substance of a k'li, and he objects to it, his immersion is disqualified. If he does not object to it, the immersion is acceptable unless the intervening substance covers the major portion of the k'li or the person, as explained.

...Her friend should not hold her with her hands when she immerses unless she loosens her hand (i.e. grip) in order that the waters will come in the place where her hands were holding. And if she rinsed her hands in the water first, it is permitted since the water on her hands are connected to the water of the mikvah.

One needs to stand over her (i.e. watching her) a Jewish woman, older than 12 years and one day during the time when she immerses, that she should see that no hair of her head remains floating on top of the water. And if she doesn’t have anyone who will stand over her, or it is at night, she should tie up her hair on her head with wool bands, or with the ribbon in her hair, provided that she loosens them, or she uses hollow link chains or if she ties a loose garment around her hair. ...

A niddah (i.e. woman in the state of ritual impurity) who immersed in her clothes is permitted to her husband. ...