Before starting your trip, you should check the car for damages. (Yup, just like you were taught during your driver's lessons.) Besides caring for your own safety on the road, this ensures that you won't be held responsible for damages possibly caused by drivers before you. No need to report minor damages, though.
1. Start the carsharing app
2. Tap on a car and start your rental
3. Walk around the car for a quick check
4. If you spot anything, like a flat tire or a crack in the windshield, tap on "Report damage"