Hi there! Well, the good thing is you seem to already have a close friendship with her. That’s definitely a start as it means you obviously know her well enough to even think about sharing your feelings. It also means that if things don’t work out, you still have a friendship to fall back on, so you can still keep her in your life.
The advice that I would give is one similar to advice I’ve seen someone else give to a person in a similar situation. You should give her little hints that you want to get closer to her, without making it obvious that you have romantic feelings for her. e.g. flirt with her a little, or go with her on a date-like outing (like going to a movie together – a romantic movie!), spend more time with her than usual. Whatever you do, make it something that can always be interpreted two ways, so that if she seems uncomfortable you can simply act like it was just a friendly gesture, and nothing more than that, and go back to the activities that she’s comfortable with.
If she does show interest in becoming closer to you and responds well to your extra attention, then find a good time to talk to her and tell her how you feel, heart to heart. Tell her that you don’t want to make her uncomfortable but that you really need to be honest, and that no matter how she feels you still want her to be in your life. If you show her that you’re being genuine and not pressuring her to act against her comfort level, then there should be nothing for her to feel threatened by.