Freelancing while being Employed in Germany

I am an expat living in Germany for 2 years now. My question is how to report one-time freelance earnings and how to register as a part-time and one-time freelancer, considering my situation:

• I am a full-time employee
• My visa permits self-employment
• In 2019 I took a side project on upWork (one-time, billed less than 2000 eur. in total)

I haven’t registered as a “freelancer” beforehand since I thought there is a way to add small additional income in the normal tax return forms. It turns out that in any case one has to be registered as a freelancer.

I am a bit worried about my situation since I have to register “retroactively” (if it is even possible) and then get a 2nd Steuernummer and file my tax return.

I would like to be clean and report those earnings, but don’t know how to do that..

Any help is appreciated.

P.S. I also don’t know if it makes sense to get a tax advisor/consultant.