P.1 Composing music

Know what the director wants:

Director might not always be experienced with the music industry as its not their job, therefore you need to listen to what the director wants and expects you to compose and do so.
He/she might just tell you the kind of mode you need to create, such as for example; young girl meets boy and immediately falls in love. You would need to be able to expose the feelings for these two characters passing the emotions from the characters onto the audience to create the atmosphere that the director wants you to create.

Experiment:

Sit there and don't be afraid of experimenting with your laptop, guitar or however it is you're going to create music. You don't lose anything by experimenting as long as you don't miss the deadline! 
You might have to create a type of music you're not used to or have little experience in therefore its good to mess around with different tones and tempos.
Sometimes the most effective music choices are the ones that you don't expect and you just don't think its going to work but you won't know unless you try new things and experience.

Immense yourself and live the product:

Watch the film for the first time normally just to get the story line and know what is happening between the character, what is the situation, the problem and the climax.
Then watch it again and pick up on details such as how they have used the sound and the editing. Then as silly as it sounds, let the film play on the background while you're doing something else even if you're not watching it your brain will get used to it.

Watch the film with no sound:

Also watch the clip with no sound and your brain might make up a soundtrack for it, but you must do this without having watched the film as otherwise you already know the originally soundtrack.
You will notice details you might have not noticed if you watched it with sound.

Know the feelings of the characters:

You need to know how the characters are feeling because music is all about emotion and even if its not shown you might have to transmit some sort of emotion from one character through the other one with music. You might put yourself into the main character's situation and think about how that character might feel in the situation and how it would react to it. That way you will start to think about what music you want to create and think its appropriate to play on the background of that scene.

Know the beat

Its really important you know the beat to the film you're about to compose the song to. If you're supposed to create and eerie atmosphere you can't have pop music or r&b music playing on the background because it would create a completely different beat. You might consider playing classical music or a lullaby such as these ones