Regarding Accounting

Hire a good Accountant with GST knowledge.

You can pass one single entry (State wise) daily for all B2C Supply with total amount and for B2B invoices you have to pass seprate entry (i.e invoice wise entry)

In GSTR 3B you have to report total Net sale after sales return (i.e net figure).
In GSTR 1 You are required to report B2C sale total figure net of sale return state wise..and for B2B invoices, reporting is invoice wise..
All this reported along with your other sale (not through e-commerce sale, both or same).

Thank u very much sagar sir for this help

You may look at accounting softwares, as technology is the way forward. I understand many of them have standard accounting entries and even a person without finance background may be able to maintain operational books.