I generaly do prime my imperial modles black which also serves as the black base coat.
Then I add junky detals of all kind like wing art, panels and the likes.
After doing the major base coats and details I simply use heavily thinned down paint (water+color) to do a general wash on almost the whole model. And maybe some basic pre-weathering.
I am thinking about getting some Nuln Oil myself to wash, cause I heard only good about it.
After washing I drybrush the whole model, exepct some details, in a subtly lighter base color tone.
When this is finished all the details get their respective highlighting either by directly painting it or drybrushing.
Then you would want to do the engine exhaust if you want to and the final highlighting and weathering.
When finished and satisfied with the result seal everything up.
This is how I prefere to do it, but maybe someone else does it differently.
EDIT: I do not have any experience with copic markers and the likes except that they tend to get a bit shiny or glossy when dried up. You wouldnt want that for dark areas that are in the shadows.
I read about useing pencils. The results realy look good. Ill definitely try that myself.