GST late registration: How to pay GST for earlier invoices

Dear Sir/Madam,

I am a self-employed professional, offering business consultancy services. In the FY 2020-21, I have exceeded 20L threshold in the month of December 2020. I got my registration in Mar 2021. From Dec 2020 to the point I got my GST registration, I have nearly 6L invoices done and got paid. After obtaining the GST registration, in the month of Mar 2021, I have invoiced 3.5L, for which I made proper GST invoices.

After researching a while, I came to understand that I now need to pay the GST on the amount that crossed the threshold 20L. That means for the 6L, I am going to pay 18% GST with interest. Now, considering the cum-tax relief (CUM TAX VALUE FOR SERVICE TAX (taxmanagementindia.com), I have made the computation for the dues (file attached)

My questions are:

1. Is this computation correct?
2. How to pay for this? What is the proper challan? Is there any return for this, now that it is Apr 2021?

Thanks a lot,

Cronus