Decide on your overall storyline. This is where you will break down who all your characters will be. The hero, the enemy, the obstacles, as all of these must be plotted out in your head before you can start breaking down your script idea.
Pick your opening image. If you go from start to finish, this is where you are going to set up the tone of your screenplay in order to most effectively get the audience involved. This image often will be active and will either end on the hero or follow the hero through something.
State the theme of the movie. Whether it is going to be a family drama or a comedy of errors, pretty early on you should make sure the audience knows why they are watching your movie. The readers who will need to take this script to the next level in order to get it to the screen need an easy way to pitch this to their boss.
Set up the conflict. This includes a little bit of back story, but more importantly shows the audience why this is going to be an interesting movie to watch. Without conflict there can be no resolution. This includes putting forth a catalyst, something that has changed to push the hero to have to act. This catalyst will set into motion a course of debate either between friends or enemies.
Prepare a secondary storyline. There must be layers to your screenplay in order to make it interesting to throughout. Secondary storylines can be anything from a love interest to a quirky sidekick, but these help break up the movie and set the overall tone. This storyline can also be a terrific way to misdirect people if they plot is too predictable.
Pay attention to the midpoint of the script. At about page 55, the stakes must be raised on the hero. This can be anything from adding a time element -- someone might die soon or the girl of his dreams is about to marry another man. This puts pressure on the hero to take action faster.
Pretend the hero has lost. This is an important part of forming a complete story. This is the part of the movie that everyone relates to, thereby further dedicating them to the cause of the hero. This defeat is where the hero will either have to talk himself into moving forward or one of your secondary characters will help push him to keep up the fight.
Resolve all of the conflict. Whether your hero gets exactly what he wants or not, you should try to close out the third act with a definitive conclusion. This can be the hero winning, deciding on a new goal or realizing his true desires. But this act should give the audience closure as to what happened throughout your screenplay.