Hi Padmaja,
You can have a look at document Step By Step Procedure to Enhance Standard BI/BW Extractor Using BADI for a good overview of the steps with a coding example.
Please also consider to implement a solid framework to encapsulate the coding for every new BAdI implementation. My document Implementing Extraction Enhancement using SAPI BAdI - Encapsulation via Enhancement Spot describes how to implement such an approach.
Best regards,
Sander