Any tips on getting grandparents to use the correct pronouns?

Just keep on reminding them. And get everyone else in the family helping too. Once everyone starts doing it they just might catch on. Just be patient and keep teaching. If you are in some kind of gay youth group or involved with a gay community center or pflag - once the pandemic calms down and there is some kind of get together or cook out for families - take them along. If they see other parents and grandparents adjusting to the pronouns it will rub off on them too.

You need to choose your battles wisely.  Getting the grandparents to refer to you as they/them instead of the him/her and he/she pronouns they've been using for the past century is not going to happen.  Maybe you can get your parents to go along with it, but as far as the grandparents go, pick a gender and stick with it.  You are a singular person.

Plz don't expect them to change. They do not understand. I am a senior lesbian and thought I was pretty open minded but I don't get it either.

I do know that grandparents are perfectly capable of they them pronouns. If it helps, tell them to pretend that you’re 2 people. That may help since they them is used a lot it as it’s plural form. 

You shouldn't expect them to understand you. You should try to understand them.

It's curious how some people want to be accepted for who and how they are, but they refuse to accept others for who and how they are.

So how's that, your not a boy or a girl but you identify as both? I would think that "it's" would be the proper pronoun.