Make sure your device has plenty of battery life.
For keyless cars please ensure the key is placed in the centre console. It is also reccomended you apply the seat belt during the process.
Put your device in flight mode (but ensure Bluetooth is enabled) so you are not disturbed by any calls.
It is strongly recommended that a battery stabiliser or at a minimum a charger is connected to the car for the first unlock or any full flash writes.
Some 2019+ cars require the bonnet open in order to enter a programming state.
Ignition on, engine off. Do not switch the ignition off at any point during the flashing process as it will interrupt communication with the ECU.
Once the flash process has completed you should turn the ignition off for at least 30 seconds before starting the car.