As a girl, I second Mr.Canadian here's idea. Try to create a little bit of a scenario first though, cuddle, playfight, whatever works for you
And then just stop at some point where your faces are close, and either look into her eyes, or if you're too shy for that, whisper into her ear and ask if you can kiss her. I know a friend of mine's boyfriend actually asked before their first kiss, and I swear all the girls were like "ZZOMMGG!!1111!!!11!!1!" when she told us about it. It's a really really sweet gesture. And if she says no, you can just apologize. You're still a gentleman
and it's not as big of a deal as if you'd thrown yourself at her and she'd had to push you away with all her might.