How can I declare tax if I have no job but investment?

Hi there, My friend comes to the UK from Hong Kong with BNO. He is retired and not working in the UK. But he got some investment here. He knows he needs to declare tax. But how? We don't know how to do so. Can HMRC tell us how? Should he first get a NI number? And then what is next? Thx.

Hi Ying Kit Wong,
Please refer to Self Assessment tax returns.
Thank you.