I've found over the years that calmly, clearly and carefully explaining to that person what they did, why it has hurt you in what way, explaining your emotions through the incident and after and getting them to see your point of view is the most instructive.
I learnt this through bringing up two teenagers mostly.
It would depend on how bad what you did was. I am a very forgiving person and it takes a lot to get me upset. However, there are things that I won't tolerate, being unfaithful, lying, stealing, etc.