Unfortunately, there's no easy way. Just ask, tentatively, if she was ever bi-curious. If she says yes, then onwards. But if she said. no, then tell her anyway. She may just be shy! Say, well, would you like to experiment?
(I recently discovered I was bi, and I asked my best friend out this way. It went great!)
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You can tell her directly and honestly that you are a lesbian and that you have feelings for her. Being open and transparent about your feelings and sexual orientation is important in any relationship. It's important to have a conversation that promotes understanding, respect, and openness.