What is the best advice you got from a beta reader or editor on your female characters?
Stop treating them like a role and start treating them like a person. Most of the times when guys write girls poorly, it is because they are saying "Well, this is the X. This person's role in the story is X," and then they make the person not exist beyond that. Every character, regardless of gender, should have their own motives, passions, and you should be able to know what they were going to do with their life if the plot hadn't smashed into them, and that can go a long way toward helping with that.
That was the big thing for me, was not writing anyone to a role... making everyone the hero of their own story. That was the big thing, but it was a process over time, figuring out treating people like characters instead of roles. That's kind of nebulous, right? Tell them to read a bunch of books by women, because a lot of them haven't, and that's part of the reason they're doing it poorly.
Oh and here's another big thing. The first way of being sexist in your writing involves writing people into roles, right? Into stereotypes. The next thing that people generally do, you'll see this a lot in cinema right now, is take the underrepresented group, or the token female or something, and make them awesome, so that they don't actually have any sort of-- they're just good at everything. Right? That's the next level of doing something wrong, and the third is where you're like, "Wait a minute. Let's make everybody kind of quirky and interesting in their own way, rather than putting anyone on a pedestal," and things like that.
And it's a process for all of us. You'll notice that like in the Mistborn books... I was so focused on making sure I had a strong female lead, that there is like no other women in whole the book. And that's a really common mistake... But you just get better at it the more you write.