When to do Self Assessment tax return forms, and how to transition from employed to self employ

I got my UTR on the 3rd Dec 2023 (no work as self employed), while I was employed by a large company with pay as you earn tax scheme. Then I resigned in January 2024, and started working as self empolyed, generated some profit, when should I fill the self assement tax return form to pay my income tax. How does it link to my previous employment and tax states?

Hi Iain.Wang,
A self assessment tax return is used to declare an individual's entire world wide income and capital gains in a tax year.  In the tax year ended 5 April 2024, you would have employment income, as shown on your P45. You will also have self employment income and expenses.  On a paper tax return, you would complete SA102 (employment) and SA103 (self employment) as well as any other self assessment supplementary page that is appropriate.  If you have not registered for self assessment, you should do so, as self employer/sole trader (Set up as self-employed (a 'sole trader'): step by step).
Thank you.