You’ll know her well. If you’re the kind of person who is self-aware, you’re bound to write a realistic character based on you. You know what you look like, how you sound, your strengths and weaknesses.
It’s easy to have her become a Mary Sue. Despite how well you know yourself, we always want to present ourselves as better than we are. It’s easy to write the character based on yourself as perfect, or write who you want to be instead of who you are. You might find yourself writing dialogue and easily saying all the witty responses you wish you were quick enough to come up with in real life. I’ve found happening on occasion in “Sent From a Dream.” Rose sometimes says out loud all the things I’m afraid to say to people’s faces. So, how do you solve the Mary Sue problem?
Write in some distinct differences. I always say Rose is me, but not. She’s loosely based on my past self.
Let your you character grow her own way. Maybe basing her on yourself is a good place to start, but if she starts to take off in a different direction, let her. She’s like your child. You’ll always see the similarities, but she’s her own person.
Play up her flaws. You might have to be a little self-deprecating here, but it’ll balance out well. We all have things about ourselves we don’t like, both physical and in our personalities. It’s not easy to bring those things out into the open, but they’re what makes us real and interesting.