Usually it will be whatever technical documentation, specifications, and design documents you have. Access to your project team members for answering questions is essential. An estimate of your project can sometimes be made by doing a cursory investigation of the requirements.
Estimates are usually only accurate to a factor of three or so.
An estimate is designed to give you a rough idea of what the project will cost. A proposal shows you how the project will be done, when it will be done, and will provide a cost that you can use for financial planning purposes. First, information is gathered about the proposed product through examining prior software, existing specifications, and by interviewing project team members at your company. All the information gathered is then analyzed and the project is broken down into smaller components which are estimated separately. Follow up questions are asked, and past similar projects are used for costing guidelines.