Know the basic details of the soap opera you will write for. This includes the setting of the show and the main characters.
Watch the show to see what storylines are currently running. You'll probably need your story to go along. If your story goes in another direction, it will need a good transition from the current story.
Create the basic storyline. The favorites usually include love triangles, heated fights within families, vicious competition between businesses and dark secrets from the past.
Create the conflicts for each character. Determine who's going to cheat on who, who's going to lose their memory or get a brain tumor, whose long-lost child is going to return (grown up in just three years) and who's being blackmailed by a mysterious stranger.
Determine how long the story will last. A good one will take weeks, giving only tidbits of new information in one day.
Write the story outline for each day's show. These are known as "breakdowns." One outline usually runs fifteen pages long and is detailed enough to list each scene's time of day.
Write the story script. Remember that it should be in screenplay format, using the correct script elements and scene heading. Read up on the rules of screenplay writing for this.
Submit your story to the producers of the show and hope for the best. The worst they can do is just say no.