A. Express the emotion: Whatever the crime or injustice or violation, the forgiver needs to fully express how it made her feel. If the transgression elicits anger or sadness or hurt, those feelings need to be deeply felt and expressed. If it's possible to express it to the perpetrator, great. If not, a stand-in, empty chair, heartfelt letter or yelling in the car with the windows rolled down might suffice. Are you expunging all the feelings? Probably not, but enough to allow you to focus on the other areas.
B. Understand why: Our brain will continue to search for some explanation until it's satisfied. Maybe we won't agree with the rationale, but we need some schema that explains why the act took place. In some situations, even an acceptance of randomness can be a sufficient paradigm.
C. Rebuild safety: The forgiver needs to feel a reasonable amount of assurance the act won't recur. Whether it comes in the form of a sincere apology from the perpetrator, a stronger defense against future attacks or removal from that person's influence, safety needs to be re-acquired. To a reasonable amount, of course, because we are never 100% safe.
These three elements help us process the event. It's how I feel, how I understand what happened, how I know it won't happen again. On to the fourth:
4. Let go: This very difficult step is a decision. Letting go is making a promise to not hold a grudge. In the case of a relationship, it means one partner won't refer to that past transgression again: "I'm forgetful?!? Well, you forgot our anniversary once!" It's resolving to refrain from lording the transgression over the other in the future. When it comes to forgiveness, the victim holds all the power. I've even seen a smile creep over the face of someone who has been trespassed upon: "You screwed me over? That gives me a whole year of guilt-tripping." Letting go means surrendering this dominant role; a stepping down from the powerful position of victim to allow equality again. In addition, letting go is making a promise to yourself that you'll stop dwelling/replaying/ruminating/perseverating on the injustice. If letting go feels impossible, it's probably because A, B or C weren't sufficiently completed.