Basically the best time to do it is when you are alone. What I did, was that I just kissed her a few times, then you just open your mouth when you are kissing. It comes naturally, and should be fine. Just make sure you haven't got anything stuck in your teeth from lunch!
Also, it could be helpful if your leaning against something. For me, I was just standing up, and by a tree by a river (in private). If you know any good places, then go there. I'm almost certain you will remember it in the future!