Make sure your profile is complete before you make your first booking (see "How to set up your Europcar On Demand account").
Open the app and enter the booking period. Use the filter function to narrow down your search.
In the map you will now see all available vehicles. Tap on your desired car, add any additional options (such as Europcar On Demand-Safe) and confirm your booking.
You will see your booking information on the app and will be sent a confirmation email.