Nobody ever fell in love without being a little brave
Just gotta work up to ask. For me, I started by asking her to come to a group outing with me. No, not the sleezy tactic of "Totally going to be a group" then it's just me, but actually a group outing. My friends knew I liked her and they tried to sit so we'd be next to each other. Couple of weeks I worked up the courage to finally ask her to see a movie with me, and she said yes.
At the end of the day, it's cliche but the worst that would happen is "No." If it's a no, it sucks, but accept it, and move on. It's done, it's over. No maybes, no what ifs, just, hey she's not into you, and rejection is never fun, but it happens to the best of us. If she says yes, then great!
Just remember, number 1 tippy top best advice I can give - relationships and love are not like the movies. They just aren't. She's a real person, just like you. She has hobbies, likes, dislikes, and she may be a completely different person than who you have in your head. Don't just ask out a crush you barely know. Get to know her, the real her, not the one you built up in your head. She's not going to be your Robin Scherbatsky, or your Leia, Arwen, or <>, she's not going to be the imaginary girl you imagined in your head and what you are going to do, she's a real human. I like to ground anyone who has a crush and pull them back down to earth. Crushes are fun, but we romanticize the person into someone they're not. Make sure you like them, not who you think they are.
If you don't feel ready to ask her out, like if you don't know her very well, then trust that feeling, and do what I did, try to find some mutual ground where you can get to know each other a bit more. Group settings are great for this, see how well you get along together. When you're ready, it'll still be terrifying, but like the quote says, no one ever fell in love without being a little brave.
Note: I realized I wrote this from my point of view. Swap out the pronouns if they don't apply.