What? Are you 12-years old, passing notes in class?!? Sheesh. When The Object Of Your Affection (TOOYA) comes into view, greet her warmly, happily, as you usually do. Compliment her on something. Then broach the subject with her that you would like to take her our on a Real Date. Then ask her, "Would you like that, as well?" By doing it this way, you are fully respecting her as a person. You have presented her with an honest, heart-felt offer, and then handed her the Talking Stick, so she can have full power and agency over her response. She will either smile and say that it's a lovely idea, and she looks forward to it. (At which point you should mention a place you thought they might go to eat, or a particular thing you'd like to do.) Or maybe she is not comfortable telling you no, and she says, "Ummm... welll..." At which point you can be the gentleman, and say, "I know, right? It's taking our relationship in a more serious direction, but I am interested, and am curious if you are as well." Now you two are having a conversation about your views of the world, of each other, and - as long as you remain respectful of her decisions and accept whatever she decides - this can do nothing but strengthen your friendship. And hey... maybe more will happen, too! But when you treat her directly, with honesty, and allow her to speak for herself, you have given her the greatest gift you have to give: Your respect. "I'm going to try confiding in her sister for advice..."
I'll elaborate a bit because you made the effort to type up a long comment and I didn't do the same. The direct approach is obviously the better and less schoolchildish, and I get the merits and respectfulness of honesty. But while I'm no Don Juan, I'm not entirely innocent either. Why, just last year I had a disastrous relationship with a friend! And, actually, on a serious note, that makes me a bit more apprehensive about doing anything; I'm worried abouthat this could just be another brief infatuation that'll pass as soon as it came on, and while I'm on good terms with that other friend, it upset her a lot and permanently changed the nature of our relationship. I don't want that with this friend, y'know? Neither do I want to end up hurting her or permanently have it hanging in the back of my mind when we talk. It's not just the typical ball-lessness of the friendzone. I mean there's that too, let's be real. But I'm also genuinely confused. Someone said to me once that when you get involved with a friend, it's not exactly like your relationship has "evolved", but almost like you now have two concurrent ones. And more to the point, I feel both friendly toward her and something more, and those emotions overlap and conflict in weird ways. So I'm not even sure what I want, to a certain degree. So why do anything at all? Maybe only because one cannot leave a story unturned! As you can see, I excel at justifying my cowardice.