Already completing self assessment, anyhting else to do for a sole trader?

Hi HMRC team, I'm currently employed and complete a self assessment, due to foreign income. I might want to explore in addition some work as a sole trader beside my employment. Do I need to register anyhting else with HMRC? If yes, would you please have a contact for me? Or can I just start it with anyhting else to do, as I already complete an annual self assessment? Many thanks in advance hermest

Hi hermest,
Please refer to guidance here:
Set up as a sole trader
Thank you. 

Thanks for your response. I did have a look already through the guidance, however, it is not clear to me, if I have to do anything else. As said, I'm already submitting a self assessment since many years. Can I just start a sole trader business and add it under self employed in the next assessment? thanks again hermest