Good question. I’m not sure either (which is why I haven’t done it yet…) - it’s just what I was told by more experienced forum members in other posts, such as here. (I also see that you’ve also been looking into this situation.)
I imagine that it means telling the tax office that you’re doing freelance work somehow (not so unusual I think - I remember having to do that in Germany ages ago). I got the impression from earlier reading that it wasn’t quite as complex as opening a company, just registering yourself as having self-employment income…but again, I don’t really know.
I’ve been intending to go to the tax office for their free consultation thing to ask them what one needs to do in this situation, but haven’t gotten around to it yet. Worst case scenario is that I would just report the income next year, see what happens, and hope I’m not in too much trouble. I’m also thinking about starting a company at some point, just didn’t get the chance to look into the best approach yet.