How to prepare a gap analysis document for existing BW system and fresh BPC implementation.

I need to prepare a gap analysis document. There is an existing BW system running and for implementation of BPC we need some fields which are not available in BW. BW is implemented on SD,MM .

In BPC we need sales planning, logistics planning, opex planning , capex planning , FM- BCS , COGS/COGM reports.

Please suggest me any idea how to proceed.