When in doubt, mimic her mannerisms to make her feel more comfortable. What that basically means is to follow her lead, so that she feels you are two of a kind.
But let's get more complex. People naturally become more comfortable and relaxed if talking with someone who is “mirroring” their behavior. You can literally, mimic their posture, body movements, etc. to help develop the trust and comfort they have with you.
Don't be so obvious that she notices, but do hold your posture the same, and duplicate her movements if possible. The similarities in your physical positioning will activate her subconscious mind, and that will naturally create a common comfort between the two of you.
This is critical for setting the stage for kissing! You want to be in synch with her preferences about certain things, of course without completely changing your personality.
So if for example, if she is fastidious about cleanliness, you should be the same, if she eats neatly with knife and fork, you do the same. And if she kisses with her lips tightly closed, well then it's obviously not the correct time to try a French kiss.
Other than that, don't stress too much or try to have everything mechanically perfect, go with the flow, kissing like making love is instinctive. Let yourself go and you'll be surprised at how easy it is