The purpose of a proposal is to determine the general direction of a project, not all of the little details. The first step after a proposal has been signed is to summarize all of the features in a technical requirements document. This is a multi-step process which needs to involve the client. I wouldn’t be doing him any justice if I tried to do this all by myself.
For more information about how I work on my projects, please read the following blog post: The 8 Pillars For Building Complex Web Applications