Get a Steuerberater. IIUC they charge based upon your revenue and it’s certainly easier than doing it yourself. They’ll generally deal with all the bureaucracy.

Don’t use platforms like ODesk/ Fivvver to find work as you’ll be competing on price for commodified work. You’ll have more interesting and better paid work if you have a hand in developing the role with your clients.

You need to do some sales and marketing work - e.g. create a portfolio and go to events to meet people in the industries you’re interested in. Finding work is largely luck but you don’t invest the time then you’ve got no chance.

The fees will really depend on experience and the value you’re adding. Check out e.g. Glassdoor for equivalent employed roles to get a ballpark. Estimate 200 days per being available at full utilisation.