The first draft of anything is probably not good so don't beat yourself up over your first draft. When you are writing your script you can do it anytime of the day there is no rule when it comes to timing. But it is best to write when you are more focused. If you write a few pages a day before you know it you will have a 120 page screenplay. One of the important thing that you should ask yourself before you begin writing is, is my story best as a short story, a movie, a play, or a book?
See you again soon for more on script writing....