At the end of the 4th chapter the Chofetz Chaim details the process of repentance for someone who has spoken lashon hara chas veshalom. If the listeners did not believe or accept his words, then his aveirah(transgression) was only between him and G-d. Therefore, he would follow the normal procedure for teshuvah as with all mitzvoth bein Adam leMakom – regret (charatah), confession (vidui) and a resolution not to do the sin again (kaballah le’atid).
However, if his words were accepted and/or believed, and as a result the subject of his words was lowered in the eyes of listeners to the extent that he incurred a certain amount of damage, whether to himself or to his property, or if he suffered emotionally (embarrassment etc.), then the teshuvah procedure follows that which is normally required for an aveirah bein Adam lechaveiro (between man and his fellow). In this case, even Yom Kippur does not atone for the sin until the speaker appeases the subject of his lashon hara and gains forgiveness from him. Once the speaker has gained forgiveness, he then has to follow the procedure of teshuvah between man and G-d as detailed above.
The Chofetz Chaim adds that if one spoke lashon hara about another and it has not yet caused him damage – but it probably will in the future, the speaker should make every effort to convince the listeners that he had been mistaken, in order to prevent that damage from ever occurring. If he succeeds, again he is left with teshuvah between man and G-d.
As the Graz writes, to gain forgiveness from one’s fellow man is a prerequisite to teshuvah between man and G-d, and hence, despite all of the Selichot and tefillot during Elul and the Yamim Noraim, which focus on gaining forgiveness, one must not forget that gaining forgiveness from one’s fellow comes first and to try to gain forgiveness from G-d without first appeasing one’s fellow does not work.
The Ben Ish Chai (Vayelech 6) emphasizes that if, chas veshalom, one spoke lashon hara about his parents, there is a particular obligation to gain forgiveness from them, and one who does not do so is not fulfilling kibbud av v’em – he is denigrating his parents. If he is foolish enough not to request forgiveness from them, they should nevertheless forgive him of their own accord, and likewise a husband and wife should do so.