@Hector Papi Castillo Once made a video on this.
The advice boils down to:
1) Talk more than usual to get her out of her shell. If you talk less, she will probably interpret it as awkward silence and not feel motivated to respond. Of course, make your words count.
2) Make your words count by inspiring trust in you. Build rapport so she feels safe answering even if she is nervous about saying the "wrong thing". She should feel like you are attainable
3) Monitor your attainability and know when to raise it. She might be nervous that you are "too good" for her or she isn't "good enough" or wondering what you see in her. Make a self-deprecating joke, be vulnerable in a good way (ex. Share a story abour self-improvement if you used to be shy or isnecure, or drop your guard near an animal or baby), or consider a well placed-compliment. You should be attractive, yet attainable.
4) Continue to be attractive as usual to keep her invested. Shy girls are still girls; your fundamentals will still make or break the interaction.
5) After the interaction, acknowledge that a shy girl may still be passionate. Hector says that shy girls make the best lovers once they trust you, so don't think of her a static, unchanging shy girl. She is a woman who doesn't open up easily, but she has a lot within her. Once she opens up, you're in for a delight.
Watch the video, though. I'm just a rookie.