First consider romantic kissing scenes that you have seen in movies, read in romance novels, or even experienced in real life. Consider what moved you about those scenes.
Was it the setting of the scene - could it have been raining or in an airport? Was the setting important to the mood of the kissing scene? Also consider the time of day, the feelings of the two people - all of these factors do affect the scene, so make sure that you study these circumstances prior to writing a kissing scene in a romance novel.
Now set up the scene with your characters. Considering your story, where should they kiss? What will the setting be like? What time of day will they kiss? All of these little details will engage the reader in the action and lessen the actual physical writing of a kissing scene, which may become cumbersome and stale.
The physical writing of a scene in a romance novel involves giving your reader a visual of how your characters are responding to one another. Try to involve all five senses when writing a kissing scene. What do the characters smell? What do they see? What do they hear? How do they touch? Do they taste anything - the salty sea air?
If there is movement, then don't write it all out at once, disperse the movement of characters between other sensory details. Drawing out the physical movement creates more intensity and keeps the reader wanting more.
By intertwining both setting and physical movements and senses into the scene you will be creating a full picture for your readers of your kissing scene.
Learning how to write a kissing scene in a romance novel is mostly like writing other scenes in a novel - attention to detail and engagement of the reader through feelings, setting, and senses.