Familiarize yourself with the basics of the romance genre. All romantic novels center around a relationship between two people--how they meet, how their love blossoms, the obstacles they face in building a relationship, and the way they grow and change through their romance with each other. The central couple remains at the forefront: while other subplots may arise, they are secondary to the love story. Furthermore, all romantic novels end in a positive or upbeat manner. While a few embody a tragic romance, most conclude either with "they lived happily ever after" or a sense of optimism about the characters' future.
Learn the specifics of Harlequin's formula. Harlequin specializes in category romances--short books about 50,000 to 60,000 words long and fitting into one of numerous different subgenres beneath the overall romance genre. Harlequin Historical books, for example, are set amid the backdrop of history, while Harlequin Blaze books tend to be more sexually explicit than their other lines. Category romances are often published in a sequential series, with new titles appearing month by month. Harlequin has dozens of categories, both under its own banner and under Harlequin-owned banners such as Silhouette and Steeple Hill. Each has their own specific criteria, which you should understand before you sit down to write.
Develop an overall plot for your novel. It doesn't need to be heavily detailed: just a basic framework depicting the main story developments from beginning to end. All Harlequin romances are told from the heroine's point of view (though they allow for occasional passages told from the hero's perspective if it's absolutely necessary), so use that as the guide for your bare-bones structure.
Write up a draft of your novel, adhering to the specifications outlined in Steps 1 through 3. Though it's not strictly necessary, try to research any areas which play into your story. For example, if it's set during the French Revolution, try to familiarize yourself with that period. Or if it centers around deep sea diving, learn about the particulars and equipment used in such endeavors. Don't overwhelm your manuscript with details--your readers want to hear about the passionate love affair, not occupational minutia--but a few knowledgeable tidbits scattered here and there will lend your manuscript some authenticity.
Revise your draft once you have completed it, excising any unnecessary passages and polishing rough patches where the prose doesn't flow as well as it should. You can revise your novel as many times as you feel it necessary, but every novel should undergo at least one or two revisions before moving forward.
Show your novel to a close friend or and ask for suggestions. Your friend should have a good knowledge of writing and grammar, if possible, and be a fan of romance novels. You don't need to apply every change she suggests, but constructive criticism can help you tighten up any spots which require it.
Send a query letter to Harlequin, asking the staff to take a look at your manuscript. The letter should include your published works, the category you think the novel is appropriate for, and the elements of the novel which you believe make it stand out. Include a self-addressed stamped envelope, allowing the staff to reply to you easily.
If Harlequin asks for the novel, print it out on double-spaced bond paper and secure it with a rubber band. Make sure your name, address, email and telephone number are on the cover page and be sure to include another self-addressed stamped envelope, then mail it off to their offices.