I'm going to yell at my father to flush the goddamn toilet
Why not just flush it for him and move on?
He makes such a damn mess!
I understand why that would be not only frustrating, but disgusting as well. You should try to talk to him about keeping the bathroom clean instead of yelling about it. It will get you further if you take a calm approach to the situation.
The bathroom is too messy for me to take a calm approach.
He's expected to clean it up after using. You've every right to get angry at him but yelling at him is not gonna do any good so you should talk to him politely.