Decide first whether you should be the one speaking
Before anything else, sit with this question honestly. You may have been asked because you are the obvious person, the oldest child, the only sibling, the spouse. That does not always mean you should accept.
If standing up to speak about this person would require you to lie, or to hide a level of pain that is going to break through anyway, it is fine to say no. You can ask another family member, a clergy member, or a celebrant to deliver the eulogy. You can write a few sentences and have someone else read them. You can not speak at all.
If you decide to speak, decide with full awareness of what you are taking on. The eulogy is a public act. The room will hold your words long after the day. Choose what you can stand behind.
If you are unsure, give yourself twenty-four hours before deciding. Talk to one trusted person who knew the relationship from the outside. Then choose.
What an honest difficult eulogy actually does
An honest eulogy for a difficult relationship does three things, in this order.
It tells one true small thing about the person. Not a saint version. Not a list of grievances. One real human detail. The way she made tea. The thing he taught you, even though he taught it badly. The fact that she was funnier than people gave her credit for.
It acknowledges, in one sentence, that the relationship was not simple. The room often already knows. Naming it briefly, without elaborating, gives the room permission to feel what they actually feel rather than perform a feeling they do not have. "My father and I did not always understand each other. The room knows that. I know it. He knew it. I am here today because in spite of it, I want to say something true about him."
It closes with what you can carry forward. Not forgiveness, if forgiveness has not happened. Not love, if love is not what you feel. But the thing you take with you. The lesson, even if the lesson came the hard way. The trait you recognise in yourself that came from him.
That structure can carry an eulogy through almost any complication.
What to actually say
Specifics, even more important here than anywhere else. The room will be alert for false notes. Specifics protect against false notes.
Not "he was a good father." If he was not, do not say it. Say what was true. "He was a man who worked hard and who, in the way he understood, was trying."
Not "we were close." If you were not, do not say it. Say what was true. "We were not close in the easy way. But we were related, in every meaning of that word, and there was nobody else like him in my life."
The room can carry honesty. The room cannot carry pretence. Trust this.
Choose three small specific details that are true, even if they are small. The way he laughed at his own jokes. The car he drove. The thing he was good at. The way he was with his grandchildren even when he had been hard on his children.
Build the eulogy from those true small things.
What not to say
Do not use the eulogy to settle anything. Whatever happened between you, the funeral is not the place. Whatever you have not said, find another place to say it. A letter you write but do not send. A conversation with a therapist. A walk by yourself.
Do not name specific old wounds, names of people who were involved, or specific events that the family is still carrying. The eulogy is heard by everyone in the room. The wounds are theirs to carry, not yours to expose.
Do not be sarcastic. The room will hear sarcasm at a funeral as cruelty, even when you do not mean it that way.
Do not pretend the relationship was something it was not. Even one false sentence will make the room uncomfortable for the rest of the piece.
Do not make the eulogy entirely about the difficulty. The eulogy is for the person, even when the relationship was complicated. Keep returning to him, not to the relationship between you.
How long it should be
Two to four minutes is right for a difficult eulogy. About three hundred to five hundred words.
Shorter than a standard eulogy on purpose. The room is not going to be helped by a long piece. A short, true, dignified piece honours both the person and the truth of the relationship.
After the eulogy
Many people who deliver a difficult eulogy report afterwards that the act of writing and delivering it changed something. Not always for the better. Not always neatly. But something.
Be ready for that. The piece you wrote and delivered will sit with you. You may feel relieved. You may feel hollow. You may feel angry. You may feel a kind of sad that surprises you.
All of these are normal. Talk to one trusted person about what you wrote and how it felt. Give yourself a quiet day or two after the funeral. The eulogy is one act in a longer process.