A good proposal is well formatted and easy to read for the reviewers. It must be precise and contain relevant details while avoiding any extra information.
You must clearly take care of the following points in your proposal -
While describing the timeline, be very clear about specific goals you will be completing during this one month in a weekly fashion.
While we encourage fresh and innovative ideas from you people, it's upto you to make sure that you select a realistic goal considering the time duration of one month.
Proposals for previous years' GSoC participants can be viewed here