Make a list of plot points. This is a list of scenes that you want to see in the film. A plot point can be "the ship sinks" or "Jack and Jill get married." It doesn't matter when or why these events take place; this is just a list of things that will happen at some point in the story.
Write a three-sentence summary of your story. For example, "A king and queen lived in a beautiful kingdom, but the evil witch wanted to takeover the kingdom. So the evil witch took the queen to a secret forest and locked her away in a tower, forcing the brave king to go find her. The king found the queen, slew the witch's dragon and took back the kingdom."
Label each sentence "Act 1," "Act 2," and "Act 3." For example, "Act 1 -- A king and queen lived in a beautiful kingdom, but the evil witch wanted to take over the kingdom."
Arrange the plot points under the appropriate act. For instance, during Act 1, King Jack and Queen Jill get married. Or, during Act 2, King Jack's ship sinks while trying to get to the forest to save Queen Jill.
Write the screenplay's pre-draft using the screenplay outline. A pre-draft can be a beat sheet or film treatment. A beat sheet is a detailed list of scenes in order from the beginning to the end of the film; a film treatment is an eight- to 15-page plot summary. In either case, there is very little, if any, character dialogue.
Write the screenplay using the screenplay outline and pre-draft as a guide.