Choose a gloomy setting for most of the action to take place. Classic Gothic stories were set in places such as abandoned castles, manor houses and ruins. Use the setting to create an ominous mood.
Use a recognizable Gothic hero. He's a loner, and a man of few words. He's dark and mysterious. Have him kill someone in the past. Have him see ghosts and visions. Have all the women he's ever loved die inexplicably.
Create a heroine who is a firm believer in true love. She suffers from violent passions that struggle to find an outlet in an oppressive surrounding.
Create a tyrannical father-figure. He doesn't have to be a blood relative but it's good form to make him odious. In short, create a monster.
Make the hero fall in love with the heroine, or vice-versa, to add the romantic element. Unrequited love works better since most readers will instantly identify with the situation.
Bring in the jealous monster. Turn it into requited love, or else there'll be no romance. Make the tyrant hiss and prowl around the castle on moonless nights. Make the brooding hero defeat him.
Use the metonymy of the doomed. Make the wind howl and sigh, and mingle with laughter, also howling. The Gothic vocabulary of negative emotions is vast. Choose with precision.