A summary return (single page) form in GSTR-3B will required to be filed on self-declaration basis for first 2 month i.e. July and August by 20th day of next month. I.e. for the month of July, a summary return needs to be filed by 20th August after paying appropriate taxes, and for the month of August, the same needs to filed by 20th September.
Read full details in this article by CA Pulkit Sharma