How do accounting / invoicing plings (in Germany)

I earn some plings for some of my work.
Because I also work as Self-Employee in Germany, I need to put this income into my accounting.

How can I do this? Who is the invoice-reciever? (I would create an Invoice, assign “Reverse-Charge” Text into and set Tax to 0%. (Works well for Amazon-Payments).

I’ve seen today, is not up and running a webpage anymore, and I’ve read some news that it was take over by Clemens Tönnis (