How do I ask her out without ruining our friendship?

I've known this girl for almost a year now and I really like her. It took me awhile to start to like her, but I don't know if she likes me back... she tells me I'm the sweetest guy she knows and I don't know if she just wants to stay friends or maybe have something in the future... she's my best friend and I don't want to lose her..
How do I ask her out without ruining our friendship?
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