Forget the default planes, because they are lacking a lot of instruments.
For a plane with the same systems as your Citation, I would say that a summary startup would be:
1- Battery ON (needed to start the APU)
2- APU Start
3- APU GenNow you have electrical power, so you can switch on your avionics and radios (usually linked).
4- Avionics ONNow you are able to comunicate, and request clearance for taxi. If you are allowed to taxi, then you can switch the engines on.
5- Engine 1 fuel valve open (mixture and engine RPM controls have to be 100%, commands would be CTRL+F4 and CTRL+SHIFT+F4)
6- Engine 1 start
7- Engine 1 Gen
8- Engine 2 fuel valve open (mixture and engine RPM controls have to be 100%, commands would be CTRL+F4 and CTRL+SHIFT+F4)
9- Engine 2 start
10- Engine 2 GenNow that you have real power sources, you don't need the APU anymore.
11- Disable APU Gen
12- Disable APUFor the APU bleed etc... those are systems that are not modelled in your plane anyway, so having the above procedure applied would already be good

By the way, this is the sequence I use to start my A2A Boeing 377 with Accusim. Sure it's a prop plane, but that gives an idea about a "realistic" sequence
