Most Helpful Girl
There's no need to delay. Once you're alone and have made your crush comfortable, you should just tell her how you feel. The sooner you do it, the better, and the less likely you'll be to make yourself more nervous or likely to ramble on about something unrelated.
Just be simple and direct. Say, "I wanted to tell you that I have a crush on you" or "I really like hanging out with you, and I want you to know that I have feelings for you."1