My father-in-law passed away recently. I knew him for many years, during which time I disliked him and was critical and unhelpful to him. Now, I understand what he was going through; how wrong I was to be judgmental and that I should have been helpful and understanding. How can I make amends to a deceased person? What can I do to soothe his soul? What can I do to obtain forgiveness?
How often do we wish that we could turn the clock back and change our behavior, because in retrospect we understand things so differently? We cannot turn the clock back, we can only move ahead. What is impressive is that you admit your mistake and you wish to make amends for it. Your father-in-law is now in the World of Truth and as such can see beyond the pettiness of our world.
I would suggest that if he is buried near where you live that you visit his grave and ask him for forgiveness for your past conduct. Speak openly and honestly and with a humbled heart. Ask for him to pray on your behalf and on behalf of your family. If at all possible, take along a minyan (a quorum of ten adult Jewish men) to the cemetery, and ask for forgiveness in their presence.
I would also suggest that you do on his behalf things that he no longer can do. I am referring to mitzvot, which -- no matter what his attitude was while alive -- in the World of Truth he appreciates their real value. He can no longer do good deeds on his own behalf, but you can do it in his merit and he will greatly appreciate this. You can choose any good deed, whether it be giving extra charity, or extra carefulness in any of the mitzvot--kosher, mezuzah, Shabbat candles, Family Purity, or whatever it is that you choose. You might choose something that might have been especially meaningful to him during his lifetime. This is something that he will eternally appreciate and will be meaningful for him and can atone for your past behavior.
Wishing you well,
Chana Weisberg for Chabad.org