In order for the immersion to remove the woman's niddah status and allow for the resumption of sexual relations, the water of a kosher mikveh must come into contact with every part of the woman's body. Therefore if there is any such intervening substance, (hasisa), such as dirt, bandages, jewelry on the woman's body they must be removed before the immersion*.
1. Before going to the mikveh a woman must shower and make sure that she is clean and that her hair is knot-free since these prevent the absolute permeation of water.
2. The woman must wash her armpits and any other covered places with water.
3. She must comb her hair very well so that no hairs are stuck together.
4. She must also check her whole body to make sure that nothing dirty is stuck to it. This examination should take place just before the immersion.
5. Shampooing may not be done in cold water since cold water knots the hair, the water may be warm or hot.
6. This cleaning should be done close to the time of her immersion. The best suggestion would be to start bathing and shampooing while it is still daytime in order to finish her preparations around night time, then she should immerse after sunset.
7. If she did the preparations at home it is suggested for the woman to comb her hair again at the mikveh.
8. When it is otherwise impossible to bathe and shampoo before night time, like on Saturday night, the woman may do the preparations at night as long as she does them thoroughly and not in a hasty careless fashion way in her desire to quickly resume physical intimacy with her husband.
9. If a woman must immerse on Saturday night it is suggested that she bathe well on Friday and cut her nails and do any time-consuming preparations then so that on Saturday night a minimal amount of preparation is required.
10. When a woman must immerse on a Friday night she should do all of her preparations during the day. It is suggested that all of the preparations which can be done early in the day on Friday be finished as early as possible, this is because in the rush before Shabbat a woman might not properly prepare for immersion. Showering and shampooing should be done right before Shabbat since these need to be done just before immersion.
11. If a woman must immerse on a Saturday which happens to be a Yom Tob (holiday), or if she must immerse on the second evening of a holiday which is Yom Tob, she should comb her hair well before Shabbat or Yom Tob and keep it tied the whole time until she immerses in order to make sure that her hair does not become knotted.
12. Under such circumstances the woman should be careful not to get sticky or dirty things on her, like food. She should wash her hands whenever they get dirty so that substances will not be stuck to her when she immerses.
13. When this woman finally immerses she should check her whole body and her hair well to make sure that nothing is stuck to her that will stop the complete permeation of water.
14. When a woman immerses on a Shabbat which follows Yom Tob or on Yom Tob she should wash all of her covered places, eg. underarms or under the breasts. She may wash with water which was heated on Yom Tov.
15. If a woman did not bathe, shampoo, and comb her hair, even though she checked her body, her immersion is invalid. If she combed her hair but did not check her whole body the immersion is invalid.
16. After a woman has bathed and combed she should be careful not to deal with any
sticky substances before she immerses since if she immersed with anything stuck to her the
immersion is invalid. A woman should therefore check her body before she immerses. If she
immersed without checking, her immersion is invalid and she must do it again.
* The laws of this chapter are adapted from Shulkhan 'Arukh Yoreh De'ah chapter 199