I think what has worked best for me was getting to know some of the things to do that makes the voice more feminine, and then spend a lot of time doing the things while recording myself so I can immediately hear if I did it right and what happens if I combine them.
It kinda sucks that it requires actually listening to the voice tho 🥲
I don't think I've seen any "ask transgender" style places on fedi, but this this page is a good starting point https://transequality.org/issues/resources/supporting-the-transgender-people-in-your-life-a-guide-to-being-a-good-ally