You don't "train" them, you try to help them understand sexuality.
How do I train my family to accept homosexuality before I come out to them?
Your family are not dogs and therefore don't require "training" I'm sure they have their own views about homosexuality, as most people do. I don't understand why you feel the need to "announce" that you're gay, I don't feel the need to announce to my family or anyone else that I'm straight......
I understand why you want them to get used to the idea of being gay. They are your family and you are afraid to lose them.
I hope they will be supportive. But based on your tone, I get that the chances that they will be is very slim. Tell them that it is not a health problem (believe it or not many people think being gay is a health problem), tell them it is not a disorder, and also tell them that it is not a choice! Tell them that it is just they way you are and also tell them that you are not sorry for who you are. Tell them there are lots of successful people out there who are gay. If they are religious, tell them this is the way God created you! (This is the card I play when I argue with religious people about homosexuality;-D)
Introduce them to some really awesome gay people. That's how I was cured. I had a really funny and snarky gay co-worker who opened my eyes to acceptance. Now I'd straight up punch somebody in the face if they mistreated said co-worker around me.
I'm not sure I agree with all this telling people about your sexuality. So you're gay. So what? No need for a big announcement. Just live your life.
You cannot train anyone to accept you being gay. You cannot train someone's emotions and feelings. You can't control how they feel.
I married a black man way back in the day. My grandfather could not accept that. My husband and I went to visit him and his wife. He came out to the car and spoke to my husband. He apologized for the way that he felt. My husband said - I understand where you're coming from - no need to apologize, sir. My husband asked if it would be okay if I came in by myself for a short visit - my grandfather said yes.
Moral - respect each other's feelings. You're going to encounter some type of prejudice all your life and it won't always be because you're gay.