Because you forgive it doesn't mean that you think what the person did is okay. It doesn't mean that you have to start hanging out with that person again, or that you have to tell the person you forgive, or that you forget about what happened. And it doesn't mean that you relationship with that person goes back to normal or that you don't have any feelings about what happened.
So you forgive by thinking about what happened and how it made you feel. Figure out what this situation taught you about yourself, how you became a stronger or better person because of it. Whatever the other person did, it happened for a reason that came from within that person - all people make mistakes and think at the time that they had good reasons to make the mistakes.
You don't have to tell the other person that you forgive, though you can if you want to. You can just say it privately. You do it because you respect yourself and because you have a right to get over what happened and to move on