Query on Backdated Voluntary Class 2 NI Contributions

I recently looked at my National Insurance record and have 15 years of payable gaps, currently all at Class 3 rate. For a number of years, I have been working for myself doing online surveys, and have never registered to become self-employed, because my earnings were always well below £1,000 each tax year (note - this has been my only source of income). However, I have taken on more work recently and have gone over £1,000 in earnings. It was whilst I was researching how to register to become self-employed, that I discovered that self-employed people can pay voluntary Class 2 NI contributions if their profits are less than £6,275 per year, which mine definitely have been. My wish is to pay for 5 years of voluntary Class 2 contributions commencing tax year 2018/19 until 2022/23. I believe that in order to pay these voluntary contributions, I need to be registered as self-employed, even though my earnings have been well under £1,000 each tax year. If this is correct, when I go to register as self-employed; firstly, do I put my start date as 6th April 2018 and secondly, will I then be sent backdated self-assessment forms for the 5 tax years that I wish to make class 2 contributions for? If this is incorrect, then please can you advise as to what steps I need to take? Any help would be greatly appreciated, as I feel quite confused and overwhelmed as to what to do.

I am urgently needing to register as self employed, as I need to do a self assessment form for tax year 23/24 (because of earning over £1,000), so would really appreciate an answer to my query above. Many thanks

Hi,
You should put the date your self-employment began when you register.
You will not be sent or need to fill in tax returns for years 18/19 19/20 20/21 21/22 or 22/23 if the reason you were not registered Self-employed is because your earnings below £1000 a year. You can register Self Employed online.
Please find the link below:
Check how to register for Self Assessment
Once you have registered. You can contact the National Insurance Enquiries helpline and ask to pay voluntary class 2 for the past tax years.
We will provide you with the amount, bank details and a reference number to pay to.
Please find the contact details below.
Number
0300 200 3500
Text Relay
18001 0300 200 3500 (Text Relay)
Overseas
+44 191 203 7010
Open
Monday to Friday: 8am to 6pm
Closed on Saturdays, Sundays and Bank Holidays.
Web Address
www.hmrc.gov.uk/ni
Thankyou