The first Pre-requisite to write a BBP is to understand AS-IS and map that as TO-BE(which will be ur SAP system). you need to write the following procedure's like how you are going to Map client AS-IS data into SAP and project them as TO-BE in ur BBP. Then u can analyze the GAP's which don't fit in std SAP system and make a separate column in BBP as GAP Analysis.
Make sure u define RICEFW (Reports, Interfaces, Customizations, Enhancements,Forms,Workflows) in BBP. Provide a detailed overview on ASAP methodology in ur BBP. provide overview's on each T-code & Infotype u use in SAP, so that they understand how u r mapping their data & why this specific one is used in SAP.
Hope this Info will be helpful
Mu suggestion is to first collect AS-IS document from your client and go through it indepth. Better to read it atleast 5 times before you start to write yours...
Divide the total document module wise. or better to maintain module wise word documents and later you can combine them to one doc.
Keep your questions module wise....
make a note of integration points (sub-module & Cross-module)
Analyse GAPS atlast.
Am just suggesting on how to start... Once u start u get more doubts...
If client don't have AS-IS, then start preparing a questionnaire (QA Data base).
I hope BBP documents are confidential and client data will be there in it. So, could not share with you. Sorry for that.