Read the script several times so you can pick out all the items that are necessary for each scene. These should have been picked out by the production manager who did a tentative schedule for the film, but it never hurts to check again to make sure he didn't miss anything.
Open up your budgeting software. Fill in the prices for all those things that are already established, such as the director's, writer's and lead actors' salaries. Then fill in all the people you will need--all the cast roles, crew, equipment, locations and special props or other items.
Research the locations and work with the location master to determine the pricing for these places. Then find the average rate for each crew member, call for rates from equipment rental houses and fill in these prices.
Look to the schedule to determine how many days you will need each crew member, piece of equipment and so forth. Create columns in your software for each item and its day rate, weekly rate and overall price. Calculate the totals. Have the system run reports on several different aspects: pre-production, production and post-production.
Print out all the reports on budget and give it to the producers. Save everything, because things will change as some costs are cut or added when days are extended and so on.