A GST credit note must reference the original tax invoice (or invoices) it relates to, and show your GSTIN, the recipient's details, the taxable value and the tax being reduced. It is reported in GSTR-1 for the period in which it's issued, and it reduces your output tax. The recipient reverses the matching input tax credit.
There's a deadline. A credit note for a supply must be declared by 30 November following the end of the financial year in which the original supply was made, or the date you file the annual return for that year, if earlier. After that, you can still settle the amount commercially, but you can't reduce the GST already paid.
Post-sale discounts need care: GST can generally be reduced only if the discount was agreed before or at the time of supply and can be linked to specific invoices. Other discounts are often given through a commercial credit note that doesn't change the GST. Check with your accountant.