Share your insights or perspective on the financial matter presented in the input.

Self-employment alongside full-time job
What you need to do is register as a sole trader. This will automatically register you for self assessment so you don't have to do that separately. For a simple business like you describe that's it.  Completing your self assessment will take care of all your income tax and national insurance obligations (although as mentioned in your previous question there shouldn't be any NI to pay if you're only making £600 or so a year).