Turn the ignition on

Put the key in the engine and turn it clockwise. 

Starting the engine on a flatbed tow truck with an automatic transmission should be similar to starting any other vehicle with an automatic transmission. Here are the basic steps to follow:

1. Turn the key in the ignition to the "on" position. This will power up the vehicle's electrical system and turn on the dashboard gauges and lights.
2. Press on the brake pedal and shift the transmission into "park" (P) or "neutral" (N)
3. Turn the key to the "start" position to start the engine. You will hear the starter motor engage and the engine should start up.
4. Once the engine is running, shift the transmission into "drive" (D) and release the parking brake.

It is important to make sure that the tow truck is on a flat surface and that the parking brake is set before starting the engine. Also, check the fuel level and oil level before starting the engine.Additionally, it is also a good practice to check the transmission fluid, coolant and power steering fluid levels. As with any vehicle, It's always important to refer to the tow truck owner's manual for specific instructions and guidelines for starting the engine and operation of the vehicle.