It depends on where you are with the girl and how do you feel about it. If you are a touchy person you will probably do it without even occuring to you. If you're not - then maybe playful touching will be the best option because many guys get really self-conscious when touching a girl and this makes it really creepy.
The best way is to begin to tease her in a really playful and friendly way. For example if you're watching a movie and there is a funny guy in it you can say "Oh. This guy will be PERFECT for you!" and smile. Odds are she will tell you "Hey!" and punch you playfully. If she doesn't it's okay - you just need to set a playful vibe. Then you can tickle her, punch her playfully, hit her lightly with a pillow, play thumb wrestling or something like this. After that when you're both laughing and having fun just hug her and whisper in her ear "I must tell you something...". Then release her, look her in the eyes and say "You are... the most amazing... DORK!

" (By looking her in the eyes and telling her "You are... the most amazing" you will probably create sexual tension and then when you call her a dork it releases it - the result is a strong emotional reaction) and continue with the playful kino. This way you make her comfortable by making the touching nice and fun. By amping up her emotions you can get more and more physical until you kiss her.