How can one atone for a past sin after sincere repentance and seeking forgiveness, and is there any additional deed that can be done to ensure forgiveness?

Allah has commanded the believers to make a sincere repentance (Tawbah Nasuh) and promised them expiation for their sins and entry into Gardens (Paradise). He has also promised forgiveness to those who seek it. Repentance must be followed by righteous deeds, for Allah transforms the sins of those who repent into good deeds, and good deeds efface bad deeds. Therefore, the questioner should increase in supererogatory acts of worship, recite the Qur’an, engage in supplication (Du'a) and seeking forgiveness (Istighfar), and not despair of Allah’s mercy, for sincere repentance erases what came before it.