Asking Your Friend For Mechila: Some Important Pointers
1. If you do not ask your friend for mechila Yom Kippur is not michaper even your aveiros to Hashem! ( Birkei Yosef)
2. If you cannot remove hatred against your friend from your heart your tefilos are not accepted. (Mateh Efraim)
3. You need to ask for mechila even if your friend was angry at you and you didn't do anything to him or even if he started.
4. You should ask mechila in person and not through an intermediary. If you cannot get in touch with your friend you should have in mind that you will contact him after Yom Kippur as soon as possible.
5. Some say that even if you know your from was moichel you, you still need to ask mechila because the humility is part of the kapara. Others say this is not necessary.
6. You must specify what it is that you did when asking for Mechila. If telling your friend what you did will cause him more pain than it is best to ask for mechila without getting into details. (Rav Yisroel Salanter) (Piskei Tsuvos)