The first look at startup covered a non-ATC situation, and therefore avionics were turned off after the engines were started, and the generators were online.
But when working with ATC it's important to request permission from ATC before starting the engines. How do we use the avionics without generator power? With the battery.
In this video I explain the process of filing a flight plan, requesting clearance, and permission to start the engines. It should then be simple to integrate these steps into a complete startup.