Refer rule 20 of CGST rule 2017
20. Application for cancellation of registration.-A registered person, other than a person to whom a registration has been granted under rule 12 or a person to whom a Unique Identity Number has been granted under rule 17, seeking cancellation of his registration under sub-section (1) of section 29 shall electronically submit an application in FORM GST REG-16, including therein the details of inputs held in stock or inputs contained in semi-finished or finished goods held in stock and of capital goods held in stock on the date from which the cancellation of registration is sought, liability thereon, the details of the payment, if any, made against such liability and may furnish, along with the application, relevant documents in support thereof, at the common portal within a period of thirty days of the occurrence of the event warranting the cancellation, either directly or through a Facilitation Centre notified by the Commissioner
Before filling cancellation application you have to submit all your latest GSTR 3B and GSTR 1. For eg you are applying today your GSTR 3B and GSTR 1 for the month of April 19 should be filed.
After filling cancellation application proper office either call for explanation or if no explanation is required then he will pass a cancellation order. After cancellation order file your final return in GSTR 10 within 3 months from date cancellation.
Also refer sec 45 of CGST Act 2017
as may be prescribed.
45. Every registered person who is required to furnish a return under sub-section (1) of section 39 and whose registration has been cancelled shall furnish a final return within three months of the date of cancellation or date of order of cancellation, whichever is later, in such form and manner as may be prescribed.
Also file your annual return for FY19-20 in GSTR 9 in due time.