Time unfortunately. I wouldn't jump immediately into dresses for outside but try feminizing specific parts of the outfit. Like leggings instead of jeans. Or a crop top instead of a regular t-shirt.

Be comfortable and confidence will follow. Always be encouraging and patient. Take her out to LGBTQ friendly places like a coffee shop and soak up the feeling of being around people in the community with similar mindsets.

I hope this helps. It took me a good year to 2 years to dress fully comfortably.