Hi,
You should be able to register for 2023 to 2024 online at:
Register if you're self-employed
If you are unable to register online, you can fill in the online form, print it and post it to HMRC.
National Insurance Contributions and Employer Office Self-employment and Self Assessment Registrations Benton Park View NEWCASTLE UPON TYNE NE98 1ZZ.
Thank you.