Logistics
1. Check beforehand when the mikveh will be open (it may be different from a regular Friday night), and whether it requires an appointment. Some communities may allow women who are unable to immerse at the usual time on the first night (for example, because of distance from the mikveh) to immerse shortly before Yom Tov, in consultation with local halachic authorities.
2. It is permissible to tell a white lie in order to keep immersion private. However, it is also permissible to share information with a friend or family member if this will help facilitate immersion or reduce tension.
3. Mikveh immersion is postponed if the couple aren’t in the same location over the course of Yom Tov. However, if a couple in Israel don’t expect to be together because of army service, immersion should take place on time since they may be reunited unexpectedly.
4. If it is too logistically difficult to work out immersion over the course of the “three-day” Yom Tov, a couple may decide to delay immersion to Motzaei Shabbat. It is important to complete the shivah neki’im as usual, taking special care to remember the bedikah of the seventh day. A woman may change to colored underwear after completing the shivah neki’im, and no further bedikot are needed.
Preparation
5. Perform all preparations at home before the holiday, trying to avoid getting dirty or tangling hair before immersion.
6. It is permissible to eat meat at the Yom Tov and Shabbat meals, since there is a mitzva to enjoy the meals. Make sure to clean your teeth well before immersion, in accordance with your personal practice for cleaning teeth on Shabbat and Yom Tov.
7. For women who use makeup, immersion during a three-day Yom Tov presents additional challenges. See here for a discussion of makeup application and removal and mikveh.
8. For Thursday night immersion, begin your final pre-immersion preparations after nightfall, since you may not prepare on the first day of Yom Tov for the second day. However, you can begin final preparations for Friday night immersion on Friday, because the eruv tavshilin allows for mikveh preparation on Friday.
9. There are various halachic views regarding bathing (and using hot water for bathing) on Yom Tov and Shabbat . Those who follow the view that it is permissible to shower on second-day Yom Tov should shower after nightfall on Thursday night for a Thursday night immersion, or before sunset on Friday for a Friday night immersion.
10. Some women have the custom to rinse off on a Friday night or Yom Tov night with an extra dunk in the mikveh prior to immersion. For a valid immersion, it is sufficient to wash off any specific area that needs cleaning with water and liquid soap prior to immersing.
11. Just before immersion, gently run your fingers through your hair to remove tangles, and check yourself over, including teeth. This pre-mikveh inspection (iyun) is a Torah requirement.
After Immersion
12. After immersion, pat yourself dry without wringing out your hair or a towel.
13. Thank the attendant for being there on such a busy night!
Motzaei Shabbat
Preparations for immersion on Motzaei Shabbat begin only after Havdalah, and not on Shabbat, and should not be rushed. Trimming nails before the holiday begins may save some time on mikveh night.