Decide what your boundaries are and what you do or do not feel comfortable writing. Some romance novels are simply very romantic, others are far more erotic. Figure out where your own personal tastes and level of comfort lie.
Flesh out your characters and make them detailed, nuanced people in your fictional world. Decide what their goals and desires are and why the hero and heroine are so perfect for each other.
Decide whose point of view you will write from, the hero's or the heroine's. Writing from both can give the reader a jarring experience.
Build tension in the details and dialogue. Set the stage carefully for the romance and/or eroticism that is to come.
Write love scenes that are real and believable as opposed to cliches filled with trite phrases. The excerpts from your romance novel should distinguish themselves from run-of-the-mill romantic fiction.
Give your readers what they expect; don't disappoint. If you build up sexual tension for an extremely erotic scene, then you should eventually have your characters get to that point. If you set the stage for a sweetly romantic physical scene, then don't shock your readers by giving them something too dirty or pornographic.