I believe the number one rule in forgiving sins is to do as Jesus commanded in John 20:22 and receive Holy Spirit, who provides the strength and ability to forgive, none of us can do
it on our own.
The forgiving of sins seems to be the first power conferred upon people when they are born again. If that is so, then the forgiving of sins should be our first duty as believers, but although we have the power to forgive, its not always easy
Whenever someone hurts me, I ask Holy Spirit to breathe on me strength to forgive that person, because inside of me my emotions are screaming, you really hurt me and its not fair. Remember whatever you forgive in others, it will always be forgiven in you.