Read the script several times before beginning the breakdown. Read the first time straight through without stopping, as if you are seeing the movie. On the second or third readings, begin making notes in the script about productions needs. Stay as neat and clean as possible in your notations in the script, as you are going to fill the pages as you progress with your breakdown. Meet with the director to discuss your production plan and to hear how he or she sees the movie.
Separate each scene with a pencil line drawn horizontally across the page. At the end of each scene, note the scene number and the page count of the scene in eighths of a page (e.g.: 2 3/8 or 2/8 or 5 0/8 even). An accurate page count is essential. Most often you will attempt to shoot no more than five script pages a day (for television that number is greater). Since most movies are not shot in script order, you are placing scenes together on any given day that could be from page 1 or page 100, and your breakdown pages will tell you which pant leg is torn, whether the big hurricane happened already and how many months pregnant the lead actress is.
Use five or more highlighter markers to note the key elements of the script. Mark first appearance of speaking actors with one marker (say, yellow), all extras in another color (say, blue), all mechanical effects (explosions, wind, flipping cars) in another (maybe orange), all visual effects (say, green), all stage builds (pink) and all others of note (tan). You may have different categories, or more colors, depending on the nature of the script--for example, stunts, locations, props and vehicles. You need to be as comprehensive as possible.
Enter each of the scenes into a breakdown or scheduling program. From your manual breakdown in the script, do complete breakdown pages in your software. List each of the elements you have highlighted. Print out each scene breakdown sheet as a bible in scene order. Distribute copies of the sheets to the director and known department heads so that each can add details. Enter the new elements into your scheduling program. You now have an informed breakdown.
Use the breakdown to schedule and budget the script, preferably in that order. From the schedule, you can make a very accurate budget projection. As the preproduction period commences, key crew members can help build the breakdown, the first assistatn director and the production manager can refine the shooting schedule, and then the breakdown bible will contain every element pertaining to the production. A well-devised and complete breakdown provides a blueprint within which the creative elements (the vision, look and effect) have their best chances for success.