You should book a online Special Service after booking your flight ticket rightaway (at least 48 hours before departure).
In the first step you have to choose weather your wheelchair is manually propelled or provided by electric motors (if so battery must be removed), what you need help for and if you want to get picked up at the check-In or at the gate.
First, you need to go to the check-In Counter at the airport (most airlines allow you to do the check-In at first class), the staff calls your service and register your wheelchair for the flight.
You can choose weather your wheelchair should be chosen as a bulky luggage and your service will pick you up with one of their own manual wheelchairs from the airport. Sometimes, they are going to come to the plane with your own wheelchair and they would load it when you board. At your arrival you get picked up with your own wheelchair too.
You can also choose if you need help on the plane and they have an on-board-wheelchair if you need to go to the toilet whilst on your flight.