Involve the main character in a romantic pursuit. An audience reacts best to a hero, with whom they can identify, trying to win the love of the object of his or her desire: a best friend, a high-school sweetheart or a co-worker.
Give the hero another desire that may even conflict with his or her pursuit of love. Giving the hero another interest will help to create comedic situations as the two desires create humorous confrontation and conflict.
Concentrate on the events leading to the romance, but not so much the romance itself. If the two characters wind up in bed together, don't spend a lot of time in the screenplay describing what happens there. How they got there is more important in a romantic comedy.
Show that the two lovers belong together, even against all odds. An audience must want the two people to be together and also believe that they belong together. This is the glue that holds the screenplay together. Without it, a romantic comedy will fail.
Include a happy ending. A romantic comedy screenplay must end with the two lovers together. Few, if any, romantic comedies succeed where the lovers do not end up together at the end of the movie.