That's what it's all about, really. You do the best you can to be supportive, while knowing you'll fuck up occasionally, and because you're genuinely trying, it either doesn't hurt, or it hurts less.
At this point in my life, I've had three people in my life transition. Well, three people I was close enough to, to be involved in their process fairly deeply. I've known more tr ans people, but wasn't involved enough in the process to matter.
It isn't easy to shift your brain. You build up literal paths in the brain associating a given set of words with a given person, so you have to unlearn those while learning the new associations. That gets harder as you get older too. Well, it has for me, and it seems to for others.
But putting that effort in is what matters. It isn't that much to ask at all.