One of the most challenging things for people who are learning how to write a movie script to get down is to, well, write. Maintaining a consistent schedule goes back to the earlier comments I have made about the importance of practice - if you want to learn how to write a movie script, you have to write, and there's no way around that. The thing that you can choose, however, is how you want to go about measuring your progress, and there are multiple ways to do that.
For some people, they'll set a daily page count, and it doesn't matter what that number is - 1,3,5, - it just matters that, each and every day, you manage to meet it. It is well-known that Paul Haggis has a page count of five pages a day, and it doesn't matter when he writes them - morning, noon, or night - and then after that, he can spend the day doing whatever the hell he wants afterwards.
Some people are more time based, and will set a cutoff time during the day, after which they will not write a single word, and this is a great strategy, as it gives you a chance to recharge your batteries until the next day. My personal suggestion would be a hybrid of these two styles - at the beginning, use a page count to motivate you, and then as time goes on, start using the clock as a motivator instead of a page count - you'll find yourself writing more and more each day as a result as you're learning how to write a movie script.
So if we know going in that this is going to be an uphill battle, how do we win the fight? Simple - you have a plan going into the thing, and that plan is called a treatment. It will properly structure your story and define your characters. You can't build a house without blueprints, right? Starting a movie screenplay is no different and having a proper screenplay treatment is key to your success. If you're not familiar with three-act structure, you will need to familiarize yourself with it and plan your movie script accordingly.
If you have a great idea for a story and some strong characters for it, you can also hire a professional screenwriter to plot it for you. Without a good plan, the plot of your screenplay will not move forward.