Think about your idea. On index cards write out your main characters and little bios about who they are, what they want and their motivations. Spend a week working on this aspect so you are sure who your movie people will be and can clearly visualize them and get in their heads.
Break your idea down into beats according to the beat sheet method (See the resources link for the download and information). This is a way of taking your idea and putting it into a commercially viable format that will make sense and give your story all the arcs and transitions it needs to be good. Then write your beat sheet out into 40 beats to create a full outline. Spend your second week doing this so you have plenty of time to make sure you get every beat of the story on paper and your outline is rock solid.
Purchase or get a copy of screenwriting software. Final Draft is the industry standard, but there are lots of other brands that also work. This will save you countless hours formatting and structuring your pages. Spend a day or two getting used to the software and going through the examples and tutorials.
Start writing your screenplay based on your outline. Think of each beat as around 2.5 to 3 pages to keep you on track. Aim for at least 5 pages a day, although some days you may do more and some less. Continue writing until you reach the end. You should be on week 7 or 8 now.
Set the script aside for one full week and do other things. Then come back to and read it with fresh eyes. Look for typos, flaws and plot points that could work better. Then sit down and spend one week rewriting it to make it perfect. Have fun and enjoy the process!