How do you ask a guy for a kiss?

A sexual pause there where you have a lot of sexual tension is probably the best thing, when you are just looking into his eyes, and smile, and just allow those butterflies to come up in yourself. Hopefully that will initiate the same response in him; usually people will mirror each other in this kind of situations. If he understands the situation, he might just go for it. Obviously, if you're trying to initiate somebody else doing something, there's always that risk of them not understanding completely, or if he's really shy, he may look away and break that tension, and the moment is gone. But that's the main thing: locking your eyes together, smiling and just pausing, taking it slowly — you don't have to rush to anything.