Putting Google Summer of Code project proposals on Google Melange has been phased out. The below info is still a useful framework. To get started, you can use this template to structure your proposal, but don't worry, you don't have to stick to this template one by one:
Introduction: Your software project should solve a clearly defined problem. Before offering the solution (your Google Summer of Code project), you should first define the problem. What’s the current state of things? What’s the issue you wish to solve and why? Then you should conclude with a sentence or two about your solution. This is somewhat like an elevator pitch.
Project goals: This section should again be short and to the point, and it might be a good idea to format it like a list. You should propose a clear list of deliverables, explaining exactly what you promise to do and what you do not plan to do. “Future developments” can be mentioned, but your promise for the three months of the Google Summer of Code season is what counts. At this point you are making a commitment.
Implementation: This section can be longer and more detailed. You should describe what you plan to do as a solution for the problem you defined earlier. You don’t need to provide a lot of technical details, but you do need to show that you understand the technology and illustrate key technical elements of your proposed solution in reasonable detail.
Timeline: This section is easily overlooked, yet it’s arguably more important than the previous section. With the timeline you show that you understand the problem, that you have thought hard about a solution, and that you have also broken the solution down into manageable bits. If your timeline is reasonable and its deadlines achievable, you show that you have an actual plan on how to go from idea to delivery. With this section you set expectations, so do not make promises you can’t keep. A modest, realistic and detailed timeline is much better than a timeline that promises to move mountains. Mentors are often among the top professionals in their field, and they can easily spot unrealistic timelines.
About me: Write a few lines about you and provide contact information.
(Thanks to Teo Mrnjavac for the awesome blog article)
There are plenty of blog articles out there how to write a good GSoC proposal. Contact your mentor early and to get to know his expectations. Just copy and paste the description from the mentoring project issue won't work!