"It is your forgiveness that will bring the world of darkness to the light. It is your forgiveness that lets you recognize the light in which you see. Forgiveness is the demonstration that you are the light of the world. Through your forgiveness does the truth about yourself return to your memory. Therefore, in your forgiveness lies your salvation.
Illusions about yourself and the world are one. That is why all forgiveness is a gift to yourself. Your goal is to find out who you are, having denied your Identity by attacking creation and its Creator. Now you are learning how to remember the truth. For this attack must be replaced by forgiveness, so that thoughts of life may replace thoughts of death.
Remember that in every attack you call upon your own weakness, while each time you forgive you call upon the strength of Christ in you. Do you not then begin to understand what forgiveness will do for you? It will remove all sense of weakness, strain and fatigue from your mind. It will take away all fear and guilt and pain. It will restore the invulnerability and power God gave His Son to your awareness."
-A Course In Miracles, Workbook Lesson 62
So, what does forgiveness involve? Watch the Master Teacher of A Course In Miracles in this episode from the Illuminate Mind Training Series talk about what is required for forgiveness: "Forgiveness Is the Key to Happiness".