The GST portal speaks JSON and your accountant speaks Excel.
A GSTR-1 JSON is a nested file with one array per return table; the offline tool converts it to and from Excel section by section.
Excel silently damages GST data: it strips leading zeros from state codes, turns long invoice numbers into scientific notation and reformats dates.
The offline tool's error report is itself a JSON; open it in the tool to see which rows failed and why, then fix the source rows and regenerate.
If invoices already live in billing software that prepares GSTR-1, exporting to Excel and back adds risk without adding information.
Open the GST portal's GSTR-1 offline tool, choose the option to open a downloaded GSTR-1 JSON or error file, import it, and use the tool's view and export options to get the data
The full guide has the formats, the deadlines and the worked examples.