Make tasks out of requirements then create subtasks all the way down while putting an estimate on every task. On your way you will find r&d tasks, unknowns and

Make tasks out of requirements then create subtasks all the way down while putting an estimate on every task. On your way you will find r&d tasks, unknowns and potential high risks. You need to communicate about those to mitigate as much as you can. Then communicate and communicate some more. Meanwhile you get a good overall view on the project, it's size and the possible pitfalls. After this you can do the project in whatever methodology suits the project. Big software projects just means that the above mentioned task is dauntingly big. Constant update and refining is needed as the project moves forward and there are a lots of unknowns and lots of different people to talk to. This work needs some kind of a system which includes documenting the communications and upgrading the tasks with new input.