My wife has been an amazing supporter. The first thing she said was that I needed to get my ears pierced.
She helps me with choosing clothes, and we usually go together. I mostly make my own choices, but I run them by her for valuable advice. I am developing good taste in clothes, and often she will tell me that my selections look sweet. But if something doesn't work, she'll say something like, "Well if you are sure you want that, go ahead, but I think something like this would suit you better."
She has given me advice on hair styles. She helped me choose my first wig, and came along with me to give advice when I took all my wigs to her hairdresser to get styled.
Her role is like a consultant. My choices are mine, and she is clear about that, but she will make suggestions ("I think this skirt would look good on you.") and will tell me if I am making a mistake. She will sometimes take the initiative and suggest a clothes shopping trip. As a result, I am starting to develop a style of my own that suits me.
As for transitioning her appearance slowly, maybe start with earrings. Lots of guys wear them, so it doesn't immediately arouse suspiction. I have a set of simple silver studs that I wear most of the time. I started wearing them when I was still in guy mode and no one commented. But they really help feminize my appearance now that I am in girl mode.
Help her pick out a few items of women's clothing that can pass for male. Maybe a T-shirt that has a slightly wider neckline than a men's T-shirt, but not so wide or low that it attracts attention. Women's jeans. Underwear, of course, since it doesn't have to pass.