I´m offering my 24sqm room in the heart of Neukölln. I´m living in a big bright beautiful flat with one (really nice) flatmate, Lea. Besides my bedroom you have access to the kitchen, two bathrooms and a huge living room full of plants + balcony.
You should treat the flat just as how you´d wish other people to treat yours - with respect. If you use something, put it back in it´s place afterwards. If something gets dirty - just clean it up. All in all I´d say just leave the flat as it was when you got there :)