Don't tell her.
Come up with something that gets the two of you together, and invite her out.
And then show her. With actions.
For instance if you're standing side-by-side, you could take her by the hand. Does she reciprocate? Yes? OK, then pull her closer to you. Does she seem into it? Yes? OK, then move yourself closer to her so that you're kind of sideways body-to-body hugging.
And keep going however far you want to take it, step by step slowly enough so that she has opportunity to reject your advance.
If you have the slightest doubt about the matter of consent at any point, then just ask her "is this OK?" But most likely, you'll know without needing to ask because she'll be reciprocating everything and it'll just feel right. You could also slow down or stop and leave it to her to take the next step. You kind of just have to go through it and assess the situation in the moment.
Yes, you could simply tell her that you're interested in her, but this is redundant because somebody still has to make that first move into physicality. The knowledge that you are mutually interested isn't actually going to do much to alleviate your anxiety in this sense. A girl could be head-over-heels infatuated with you, but this still doesn't necessarily mean she wants to actually get physical or that she even wants to go out with you. So regardless of whether you have that conversation, one of you is still going to have to initiate physical contact in a way where you're looking for the other's reciprocation as you gradually escalate step by step.
If you're not interested in getting physical at this time, then let your flirting do the talking for you. It's the same concept as physicality. Again, you "can" point-blank state your interest, but that really is going to be redundant in almost all situations.