Read extensively in the Harlequin line you wish to be published in. Harlequin has several lines, each with a different focus, such as Spice, Silhouette, Red Dress Ink, HQN, Nocturne Bites and Harlequin Teen. Read a few from each category to find the line best suited to your writing style and type of romance novel, and then read extensively in that line.
Visit the Harlequin website and read the writing guidelines. Harlequin's writing guidelines includes writing tips. They let visitors know which lines they are accepting submissions for at any given time as well as other writing opportunities as they occur, including writing competitions. Writing competitions and other opportunities on the site can be a good way to get noticed.
Speak with authors published by Harlequin. Authors are usually easier to reach than you think. Many have their own websites and blogs with contact forms. Find Harlequin authors whose writing you admire and network with them to learn how they broke in and any tips they might have to offer on working with Harlequin. The Harlequin website also regularly features Harlequin author blogs with a similar “how I broke in” focus.
Follow the Formula. Harlequin is known for having very specific story formulas for each of their lines. Not every writer likes to write this way and some of their lines such as Mira are less formula-driven than others such as Silhouette. Learn and follow the formula for a better chance at success.
Following the submission guidelines, submit your completed and edited manuscript to the editor of the line you're interested in writing for. Harlequin is one of the few major publishers that still accepts unsolicited (unagented) material.