The main part is mounted onto the door lock/handle frame. The stepper I used is a Daypower S-SPSM-5V.
The small gear fits on the motor, while the bigger one is fixed on the key. Because the key is required to be inside the lock, you will need a lock with "emergency function" so you cann still unlock it from the outside while the key is pluged in from the inside.
Also you need a piece of drivebeld, I just used a remaining piece from the 3D printer. Figure out the correct length and glue it to an infinte loop with epoxy or similar glue.
Steppermotor with driver: