It’s not that difficult. First, you need to start with the right subjects: Maths with English on the side. Maths alone is usually enough, but English is a good side for slow times of the year.

Then get into an agency that does tutoring at home and employs you as self employed. The don’t pay that much, but their clients pay a lot. In my area, they paid €19 for 1.5h to the tutors, but the clients had to pay €65.

So after the pre-paid package that they booked was done, I took them on as direct clients for €40-50/1.5h. Together with the travel time (I used public transport and had the flatrate subscription) that came out at roughly €20/h. And since I was a student at the time, I used the travel time to study, so it wasn’t really wasted time either.