Here’s how I did it:
1. Assembly: I assembled the mortis engine into several components: spirit hosts, necromancer, throne and the bone, the candle thing inside the cage, and the three banshees. I also left the gates that surround the candle thing off. This allowed me to access to all of the nooks and crannies of the engine.

2. Primed everything black.
3. I painted the spirit hosts and banshees by painting the top 3/4ths of a spirit host with a dark grey, then drybrushed the bottom quarter of the model with the dark grey (over the black). Then I applied a lighter grey from the middle of the spirit host up to the top, then drybrushed the lighter grey over the dark grey but stopping before I got to the black. I then painted the top of the model with white and drybrushed white over the lighter grey section. Next, I washed the hosts in thraka green.
4. For the armor I applied chainmail, then a nuln oil wash, then spot washed with seraphim sepia and dev mud, then I created a wash by putting a variety of rust pigments in a cup and mixed it with paint thinner (I added a lot of thinner so the rush effect would be subtle). I then spot washed sections of the armor with the rust wash where I thought rust would naturally collect.
5. The bone and throne was drybrushed with a dark grey followed by a lighter grey. I applied a heavier drybrush of light grey to the throne and washed it with nuln oil.
6. For the flames I painted them snot green, then I painted the bottom ¾ with scorpion green, then the bottom quarter with yellow, then a little white at the very bottom of the flame (I’m not sure if I did the white on this model though). Then I washed the flames with thraka green.
Everything else is pretty straightforward. Assembling this thing was a pain though. If I do another engine (which I probably will) I’ll make sure to place tape or something over the places where the throne and spirit hosts glue together before I paint them. That way, I can remove the tape and the pieces will bond together better.
I hope that helps!
Scott