Self Employed/Taxes and Clueless!

I was hoping for some advice from anyone that knows about being self employed and taxes.. I will be receiving SMP until February but with the cost of living we are really struggling to make ends meet with an EXTREMELY high mortgage rate.. my partner works and we don’t qualify for any kind of benefits or relief. My current employment are happy for me to work another job self employed whilst I’m on leave if I need to so the question is.. how do I go about that? I imagine I won’t earn enough in the 3 months to exceed the £12k that requires me to declare but honestly I’m totally clueless. I’m likely going to go self employed for a cleaning agency and pick up some evening cleans whilst my partner can watch the baby.