GSTR-3B is a summary return for declaring GST liabilities and claiming ITC.
It is filed monthly (or quarterly if under the QRMP scheme).
Filing is mandatory for all regular taxpayers, even if it is a Nil return.
The process involves logging into the GST Portal, entering/verifying summary data in the relevant tables, adjusting ITC, paying any remaining tax via challan, and submitting via EVC/DSC.