The weathering is really pretty simple
I do a light application of the brown first, then black after it dries thoroughly.
The brown ( "Burnt Umber" ) is Cheap Wal-mart acrylic craft paint ( yeah, the little bottles in the crafting section) & Windshield washer fluid (yeah, like you put in your car)
mix it together (mostly paint) , apply it with a small paintbrush, wipe it off where you think it's too dark with a soft cloth.
I apply heavier paint in the areas where I think the dirt would accumulate (i.e. cracks & crevices) and lighten as you move away from those heavier areas.
work in smaller areas so the paint stays wet
make sure to start wiping before it dries & kinda dab cloth around to avoid streaking
The black is pretty much the same as the brown, except a little more washer fluid
it's more of a wash, but not quite that watery
I would post some pics, but my Computer crashed recently & I lost the pics.
& just about everything else :cry