Dear Mayur,
Enabling delta for Function module based Datasource is quite complex and need to do on your own system.
Prior get desired fields up to BW and validate BEx report with ECC tables and make testing positive. Once report is done successfully, then you can try for delta.
Although you get lot of documents in SDN, they are not sufficient enough to do practically.
Even i have have created lot of generic datasources, through FM. But didn't achieve perfect delta as SAP provided .
Best wishes, Vijay