How to invoice for a small job in addition to my 9 to 5 job?

Hello,

I am a permanent resident working and living in Germany. I work a “9 to 5” job (it’s actually 4 days per week since I have prioritized work-life balance).

I sometimes give talks at other companies or organizations. So far I have done this unpaid. But I’d like to understand how I can invoice for this. I don’t expect this to exceed a few thousand euros per year total. [One website](https://russianvagabond.com/can-you-do-freelance-work-while-employed-in-germany/) I found said if this side income is under a threshold of a certain amount, you do not need to register as a freiberufer.

So i am very interested in any information on this topic — if you have gone down the route of this side-work especially for quite small amounts. And a follow up question, if it’s NOT necessary to register as a freiberufer for under certain amounts, do I simply put my own personal steuer ID on the invoice instead of a freelance steuer number?