The objective of a phased approach to development is to break your site into individual components or features that can stand on their own. You start with a needs analysis, identifying the purpose of your site. For instance, assume your site will be a combination of an e-commerce site, library site, community site, and a product brochure site. You can proceed one of two ways.

1. Collect requirements and create a design for the entire system (all parts) and then develop each part one phase at a time

When using open source systems, you can use thousands of features simply by turning them on. You don't have to create the code for the functions you need, just activate them; saving time and money. It's pretty cool stuff. But, the benefit in using open source systems can also be a big challenge when it comes to coordination different aspects of a project. Not everything exists in harmony- even when it is easy to turn on or off- and project planning/development methodologies can mean the difference between reaching your goal and just not.

What do you do if one site requires a specific version of a module, while one of the other sites requires a different version of the same module?

Because of the way Drupal is constructed, you cannot install two versions of the same module in the sites/all/modules/ directory. Attempting to do this will lead to fatal errors or, even worse, subtle bugs that arise from certain loading errors.

A traditional waterfall method breaks the workflow down into stages. You complete one phase of the production life-cycle before continuing to the next. The names of the steps are up to you and can vary and are usually tailored to your organization's style or terminology already in use. Make it work for you. The objectives of the waterfall method are to complete one phase before moving on to the next and to have the output from each task feeding into the next so that there's a logical progression of development.

Another kind of methodology is Agile software development. It help groups of developers build software using a "work smart not hard"  as-you-go philosophy: efficently and effectively delivery pieces of the site as they're completed.

This set of principles encourages the site owner and the development team to work together. It promotes team development. The Following are AGILE MANIFESTO’S 12 PRINCIPLES that you will embrace if adopting this method.

1. Customer satisfaction by rapid delivery of useful software.

RAD stands for rapid application development. With this methodology you use structured techniques, and prototyping to set your requirements, get prototypes quickly and use minimal planning. It's essentially one process that wraps several stages into one (requirements, design and possibly even the development processes.) It's close to the agile mathod but the main difference is the use of prototyping.