If you've received an advance payment for services in October but have no sales to report, you still need to account for the GST. You must calculate the tax liability on the advance, grossing it up to include GST, and issue a tax invoice. This advance should be shown in your regular GSTR 1 filing, not the IFF. The tax due must be paid via challan by 25th November 2021.
11 November 2021
You have to calculate tax on advance and pay tax while filing the return for the month. The advance received should be grossed up. This means that the advance received is considered inclusive of GST. Issue tax invoice for the advance received and show it in GSTR 1.
12 November 2021
No need to file IFF for advance. You will have to calculate your liability for the month though and pay through challan by 25th November 2021