Read what you have already. You may not have progressed to the point of a written screenplay, however, you probably have lots of notes, a general story and plot outline, as well as character sketches. Look at whatever material you have carefully, searching for common elements.
Take notes on important themes. While reading your screenplay and notes, look for important themes that run throughout the work. Make note of all these. Look for connections between them. This is the basis of your premise.
Find your statement. While you don't want to be beating your reader or viewer over the head with what you're trying to say, your screenplay is probably telling a deeper story. This is generally something very fundamental about the human condition. Ask yourself, based on your notes, what the statement of your story is.
Write down your premise in one very brief sentence. The premise of your screenplay should be able to be boiled down into a very short sentence. The screenplay premise generally relies upon fundamentals of the human experience--good, evil, love, hate, courage, violence--their relationship to one another and their broader role in the world. Don't worry if your premise sounds simplistic. It's supposed to.
Rewrite what you have. While you're basing your premise on what you already have, you'll also want to make it more cohesive, as it's something that you came up with after the fact. Go through your scenes, dialogue, character sketches, story notes, and anything else you have. Look for places where what you have doesn't fit your premise. Rewrite them to make them correlate to what your screenplay is trying to say.