How Can One Atone?

Greetings and blessings, The foundation of the answer is deep remorse for the past and commitment for the future not to commit this sin. If one unfortunately transgressed the severe prohibition of niddah relations, one must resolve upon oneself not to have relations until after purification by immersion in a mikvah following marriage. From the onset of a girl's first menstrual cycle at age twelve, she is a niddah and forbidden to all men until she immerses in the mikvah. One must be careful in all matters that could lead to sin, such as seclusion in one house without others present, and more. After deep remorse, one must confess before Hashem the sin and vow not to engage in this sin again. This is the essence of repentance. Even after repentance, one must perform deeds to fully repair the damage caused by the sin. There are several actions that if done, cleanse the sin completely after repentance: a. Observing Shabbat properly; anyone who keeps Shabbat according to halacha is forgiven, even someone who worked the entire week. b. Giving charity and performing acts of kindness. c. Being among those who are insulted yet do not insult others, who hear slander and do not respond. May Hashem help you merit complete and true repentance and accept the yoke of the mitzvot properly, and may you never come to sin again. 

Blessings, Benjamin Shmueli