GSTR

Visit https://tutorial.gst.gov.in/downloads/gstr4offlineutility.pdf

This is user manual and FAQs given by GST department .

You can file Nil return online but if you want to file return with tax that is you have Taxable supply then you need to download GSTR-4 offline utility (Download GSTR-4 Link) and fill figures of Sales and other details of business such as GSTIN and tax period. 

Then create JSON file and upload it on portal after login.

Note- You need not to provide details of purchase for Jul-Sep 17, Oct-Dec 17, and Jan-Mar 17 quarter.

Feel free to ask any doubt!