Nowadays there are two stages of the niddah time of separation. The first stage is the days in which she is actually menstruating. The second stage is a seven day period in which she is completely clean of blood. Only after the woman determines that seven days have passed without any bleeding can she go to the mikveh resume sexual relations with her husband*.
1. When the woman feels she has stopped bleeding and staining she must perform an extensive internal examination to determine absolutely that she has in fact stopped menstruating. This internal examination is called the "hefseq betahara." The seven clean days preceding ritual immersion in a mikveh begin after the Hefsek Betahara.
2. Before performing the internal examination of the hefseq betahara, it is a good idea for the woman to bathe herself, or at least wash out the vaginal area well. In any event, if it were impossible for her to wash, it is still permissible to do a hefseq betahara. In this case she should wipe herself clean before doing the examination.
3. The best time to examine herself is about 2 and a half hours before sunset, but the closer to sunset the better. In any event even if she examined herself from that morning, she can already begin counting the next day (that evening) as the first of the seven clean days.
4. If a woman forgot to do the examination of the hefseq betahara before sundown, she can still, under extenuating circumstances, perform the examination within 10 minutes after sundown, and begin counting the first day of the seven clean days from that evening.. However, one should generally be careful to perform the hefseq betahara before sundown.
5. If she normally prays Arbit before sundown, she should check before praying, since Arbit brings in the next day. However, if she neglected to check or could not do it earlier, she may check after praying. This applies even on Friday night if she already lit candles but it is not yet sundown.
6. This checking must be done with a clean white garment. If she only had a colored garment she may check with it (excepting a red or black colored cloth).
7. A Hefsek Betahara should be done by inserting the cloth as deeply as she can, reaching the place that her husband reaches during intercourse. She should wipe all the folds and crevices, and then check to see if the cloth has any impure reddish tint. A mere superficial wiping is not sufficient for the hefseq.
If she checked well in all the folds and crevices, but did not reach the above-mentioned depth, the checking is permissible, but not preferable. Likewise a woman who finds it painfully difficult to check so deeply, may check all the folds and crevices well. If even that is difficult for the woman and itself causes bleeding she should consult her rabbi.
8. A suggested way to check is that the woman place one leg on a stool leaving the other leg on the floor while she checks.
9. A woman who checked herself for the hefseq and found the cloth to be bloody, can wash the area and check again until she has a good checking. This can be done again and again as long as the sun has not yet set.
10. A woman who has a wound in her vagina and cannot get a clean check because of the wound, should consult her rabbi about how to proceed.
11. The cloth used for the hefseq checking should be examined by daylight. One can also use an electric light if the cloth was completely clean. As far as determining the color of a stained cloth, one should do that preferably by daylight. (The spectrum of normal fluorescent or incandescent light bulbs can lead to a false determination.)
12. After the clean check the most preferable method to follow for a Hefsek Betahara is for the woman to have a clean white cloth inserted in her vagina near the cervix from just before sundown until after the stars come out. This is approximately twenty minutes after sundown.
13. A woman who finds this difficult and is afraid that the inserted cloth would cause her to bleed, can do the hefseq betahara without the insertion of the cloth. She can rely on a thorough checking in all the folds and crevices which is done just before sunset.
14. If a woman had her period for only one day and is making her hefsek betahara that evening, it is mandatory for her to have this cloth inside until after the stars come out. If she was mistaken and neglected this she may rely on a check done before sunset (within 2 and a half hours before sunset) but not one done earlier in the day. This is only if she was niddah because of menstrual bleeding for one day. However, if she was niddah because of a blood stain and checked herself and was found to be perfectly clean it is enough for her to have checked in the morning to count as a Hefsek.
15. After the woman has done a successful hefseq betahara, she should wear clean, white underwear. She should also make her bed with clean white sheets.
A woman who usually has problems with staining is allowed to wear colored undergarments and use colored sheets, such that if there is a stain it will not render her niddah.
16. On each of the seven clean days it is preferable that she check herself and her undergarments twice, once in the morning and once in the late afternoon before sundown.
Even if she checked only once a day she may still immerse at the proper time. Even if she only checked the first and last days she may still immerse on time. However, if the woman neglected to check on both the first and the seventh days she must count another seven clean days.
If a woman has scabs or wounds or very sensitive skin such that the cloth wounds her, it is sufficient for her to check only on the first and seventh day. If she did her Hefsek, as discussed, and left the cloth inserted until after the stars came out and she forgot to check on the next day, the inserted cloth can count as her checking of the first day and she need only check on the seventh day.
17. If a woman was niddah because of a stain (without any other physical sensation of menstruation) and she did a proper hefseq betahara (even without the inserted cloth until after the stars come out), if she inadvertently only checked one time during the seven clean days, she can still immerse on the seventh day.
18. Although It is praise-worthy for a woman to check herself often even when she is not expecting her period, it is not recommended that she check before and after having relations because this will cause her husband to worry that she is niddah and could lead to his refraining from having relations with her. Additionally, if such checks cause her irritation and could cause her to bleed she should refrain from excessive checking.
19. If she saw even a tiny amount of blood during the seven clean days she must do a new hefseq betahara and start counting a new "seven clean days."
20. During the seven clean days blood stains found on garments or on anything else have the same rules and leniencies as any other stains. (see the above chapter on blood stains) 91. (In this law Ashkenazim are more stringent, not relying on any leniencies for the first three of the seven clean days, whereas sepharadim, treat all the seven days equally, relying on leniencies throughout.)
21. Vaginal discharge of viable semen precludes the proper counting of the seven clean days, since the semen remains viable for only 72 hours after ejaculation, the woman would not be able to count until she does a hefseq on the evening following the 72 hours after sexual intercourse.
For example, if a woman had relations Saturday night 11:30 p.m. and then had her period (any time before Wednesday night) she must wait 72 complete hours (six whole 12 hour periods) before making a hefsek betahara. In this example the 72 hours end on Tuesday evening 11:30 p.m. and if she has stopped bleeding she may make a hefseq on Wednesday close to sundown and count Thursday as the first of the seven clean days.
22. Of course, this only refers to a woman who's period has ceased but if she continues bleeding she may only do a hefsek after she has stopped bleeding.
(The above is the law according to the Shulhan Arukh, which is followed by most sephardic congregations. However, some sepharadim follow the ashkenazic custom of adding another day and only allowing the hefseq in the above example to be done on Thursday close to sunset. Some even wait seven complete days before allowing a hefseq. In these issues it is better to allow the misvah of normal sexual relations to resume as early as possible, and ultimately there is no absolute benefit in delaying the hefseq, which would thereby delay the resumption of normal marital relations. Taharat Habayit vol.2 pp. 392-393)
23. A woman who stop bleeding before six periods have passed since having relations, (for example, if she had relations Saturday night and had her period for a very short time on Sunday) may, if she wishes, take a hot bath and clean herself with a cloth, thereby flushing out any residual semen. She may then do the hefsek that evening.
24. According to the Sephardic custom, it is only necessary to wait the 72 hours if the woman has had sexual intercourse with her husband. If it happened that the woman saw blood in a situation where she did not previously have relations with her husband, she may begin counting as soon as she stops bleeding. (The ashkenazic custom is to wait five days before counting the seven clean days regardless of whether the couple had previously had sexual intercourse.)
25. If a woman found a stain which rendered her niddah, she may rely on the opinion that says that she does not have to wait 72 hours before doing the hefseq, even is she had previously had sexual intercourse (since this is a only a blood stain and not real menstruation). Instead she may do a hefsek that evening and start counting seven clean days. (Ashkenazim should count the full five days before counting the seven clean days even for a stain.)
26. If a woman made a mistake in counting and immersed one day early and had relations with her husband since, as discussed, ejecting semen cancels one's count, she must wait six periods which are four days and count one clean day then immerse again. However, if she does not want to wait these five days she may wash herself as discussed, in order to flush out the semen and then wait one day**.
27. If she immersed in the right time but not properly (due to an intervening object, etc.), even though she subsequently had relations, she may go back to the mikveh and immerse without counting the 72 hours.
28. If a woman mistakenly immersed on the seventh night of her seven clean days instead of the eighth and she had relations with her husband only on the following day, she may immerse on the eighth night since part of the seventh day past without her having relations, it is considered as the whole day and she can immerse on the eighth night without waiting 72 hours.
29. If a woman travels internationally she may count seven clean days wherever she is and need not wait until seven days have passed in the place that she left.
30. A blind woman should show the cloth of her examinations to a friend. A mentally incompetent woman should have a friend present when she examines herself.
* The laws of this chapter are adapted from Taharat Habayit vol. 1 chapters 3 and 13, and from Shulhan Aruch Yoreh Deah Chapter 196
** All Ashkenazim are strict not to do a Hefsek Betahara until the sixth day after a woman's period has started, this is regardless whether she had relations with her husband or not. Additionally, some women are strict to wait seven days, "one who is lenient will be rewarded for doing a mitzvah earlier".