Self Assessment and split year treatment for 2020-2021

I landed the UK in Oct 2020 when I had no home and unemployed. Since Nov 2020, I got my home, and I was employed until Jul 2021. I have no income after I arrived UK, I have not reported my tax return for 2020/2021. Until I got the job and did my first tax return for 2021/2022. Recently, I recognized I have to do split year for 2020/2021 on my income before I arrived UK. May I ask how I can do my split year treatment since my arrival date in 2020?

Hi ST22,
You will need to register for a Self Assessment tax return.
Once you receive your Unique Taxpayer Reference number, you can download and print off, SA100 (2021) and SA109 (2021) (split year treatment is claimed on this form) and any other supplementary pages that are appropriate to your circumstances.
From here:
Tax return for Self Assessment
Late filing penalties will be applied when we receive your tax return.  
You can appeal them by completing the appeal form here: 
Self Assessment: appeal against penalties for late filing and late payment
The address to send them to is:
H M Revenue and Customs Self Assessment BX9 1AS.
Thank you.