How to add two trade name in one gstin & pay gst for one pan

Dear Sir / Madam,

I have heard from many that I can register two different businesses with two different trade names under one GST number however no one is giving me information how to add the second trade name in my existing GSTIN and then pay the GSTR3B.

Please give me ste-by-step guidelines for the following:
1) how can I first add my second tradename to existing gstin in which I am already paying GST and filing GSTRs for my first tradename ?
2) how to pay the GST for my second tradename from starting till date with interest and late fee for only second tradename ?
3) how to file the GSTR3B, GSTR1 ?

Awaiting positive reply urgently

thankyou
Dhananjay Pathak