Register for self-employed for NI purposes

Hi All, I have filed my tax return on last Oct, and recently received a letter says " I am not registered for self employed for National Insurance purposes" and asked me to register the period of self employment last for few months and have been ended during the tax year. I had try to go thur everything in the gov.uk website, however I am afraid I cannot get the answers probably. May I have a precise guidelines where to register for this as I believe I cannot register the self-assessment again, right? meanwhile, after I have complete it, shall I need to inform HMRC again? Thanks and best regards,

Hi CK Wong,
You will need to complete form CWF1.
We would advise that you follow this link to register as self-employed for Class 2 NICs: -
You're already registered for Self Assessment
You then should click on ‘You can register using form CWF1’.
Thank you.