Start with a base coat of light olive drab. (Humbrol Acrylic)
Wash with dark brown acrylic ink. This can be quite blotchy, as it is partly to build up a scruffy weathered effect.
Dry brush with the light olive drab again.The brush should be fairly moist and contain plenty of paint to cover a reasonably large proportion of the surface.
Add some white and dry brush again. Use a bit less paint and pressure.
Add some more white and dry brush again. Use even less paint and pressure.
Finish with a very gentle drybrush that is almost white, but not quite.
Pick out a few areas by drybrushing dark silver (eg GW 'Chainmail') over a black basecoat. Ink wash these areas with dark brown ink at the weathering stage.
Now paint on a few markings in white, red and yellow. Edge the markings with black to get crisp edges. You could also print out some symbols and stick them on.
Add some more areas of dark brown ink. Water it down, blob some on, and then smear it with your finger tip. You can also paint some small vertical lines underneath to represent water stains.
Use a little chestnut ink for.some rusty patches. If it's looking too orange then mix in some dark brown.