I think you should tell her now. I don't like the idea building a friendship but secretly hoping for something more. There is nothing more awkward than a friendship with ulterior motives.
Be honest. Say what you want. There is nothing wrong with telling a woman that you find her attractive. If you don't do this, you won't ever have a relationship. If you want to date her, then tell her honestly that you want to date her.
Don't make a big deal about it, and make sure that if she doesn't want a romantic relationship you still want to be friends. She may say "I like you too". She may say "I just want to be friends". Either way you will know and you will have been honest. And if things go well you will have a girlfriend!
Ask her out in an appropriate way (I am assuming you are a teen). Get dinner, see a movie, whatever you young folks do these days.
I don't see how "taking it slow" benefits anything. Dating comes down to one person telling another person, "I find you attractive and I want to date you". If want to date her, then tell her.