Finding the Right Project
Each organization will have a project Ideas Page linked to from the official Google list of accepted organizations. Browse the list of project ideas for each of the organizations for which you are interested. The ideas should give you a clear sense of the range and depth of projects being targeted and the expectations in terms of prior experience and programming skills. In addition to the list of project ideas, many organizations encourage original ideas proposed by students.
ShortlistThere's a big sea of projects out there for GSoC. Start by compiling a list of potential project ideas that catch your interest. For each idea, take some time to carefully consider what is being proposed, how its scope might be better defined, and how it might fit in with the larger picture.
Ask questionsConsider any questions you might have about the project, how it might be implemented, and what it would entail. Read the FAQs. If there's still anything unclear about the project and requirements, get your doubts cleared. Formulate your questions and suggestions regarding each idea into a clear and concise communication.
Evaluate your optionsOnce you've researched the shortlisted projects and got your questions answered, re-evaluate your options before writing the proposal. Also think about whether the communities you're interested in match your needs, and if you'll have fun working with those people. Is there enough documentation and help available to get you started on the project? Given all the feedback, do you think you're really excited about the project and think that you can do a good job of it?
From Project Idea to Project Plan
You've narrowed down your search of organizations and projects, you've made first contact, and you've started communicating directly with potential mentors. Now it's time for the critical process of turning a project idea into a project plan.
In most cases, your potential mentor(s) will have lots of ideas and preconceptions about each project that were not included in its original description on the Project Ideas page. Discuss and research the project idea in as much detail as possible. You may even consider preparing mock-ups (illustrations, powerpoint, or web sites) to help clarify your understanding and vision of the project. You will want to discuss the scope of the project idea, including which parts are critical versus optional for the summer timeline. This process will directly feed into your application and ideally distinguish it from all the others. Just think about it: if you've helped clarify the project idea and contributed to an actual plan of action, it makes it an easy process for mentors to evaluate your proposal and give it a high ranking!
Pro Tip: The earlier you apply, the better. Submitting your proposal early helps you get early feedback.
Don't be that guy: Cut and pasting an idea from the organization page and turning that in as your project's description is a no-no. You'll be expected to research and submit your own ideas about how to accomplish the project your way, not just state the end result.