You need to log each work-related journey for a continuous 12-week period which is representative of your annual work-related kilometres travelled.
Record the odometer reading in your car on the first day of the 12-week period.
Each day, record every work-related journey during the 12-weeks showing the odometer reading when you start your work-related trip and the odometer reading when you finish that trip.
Record the odometer reading in your car on the last day of the 12-week period.
Congratulations! You can now determine your business-use percentage and keep it for the next five years.