It's a company specific question....
However roughly speaking.
As soon as the flight deck crew are aware of a problem that may lead to an evacuation a PA will be made "Attention crew at station"
The aircraft would be brought to a stop the Captain would decide if an evacuation was required, if so the checklist would be actioned. (resulting in the engines being shut down)
Then a command would be given to evacuate (this depends on the airline exactly how it is given)
That would be it the evacuation has started and the cabin staff will make sure all passengers leave the aircraft promptly and safely.