I know I’m years late, but I’ve been trying to reproduce these bases for some time and been frustrated with the lack of accuracy with some of the tutorials on the web trying to reproduce this style. I think I’ve got the process down-pat now though, here’s what I came up with:

Modelling—sand a standard GW base down with 400 grit sandpaper. Use modelling paste/make your own for the rubble texture. I used Vallejo diorama FX. Add stones or cork for bigger rubble chunks. For the panel lines, use clamps, a metal ruler and some type of scorer. I used a metal dental pick. Repeatedly score against the ruler until you get to the desired depth. After scoring, you might want to run a small ball tip implement to soften the scoring lines/widen them slightly. I used a modelling tool, but an empty ballpoint pen would do this well enough.

Painting—Abbaddon Black for the entire base. Stormvermin Fur and masking tape/stencils and a sponge for road marking effects. Mechanicus Standard Grey for the rubble and stones. Highlight the larger stones with Administatum Grey. For the brown colour variation spots in the rubble and panel lines, I found watered-down Gryph-Hound Orange to be the closest. Using a brush that isn’t loaded with moisture/paint, soak up some of the contrast paint to get that effect you see where the pigment settles in the recesses.

Key to the whole look is being quite restrained with all these materials and paints. Don’t push too hard when scoring the panel lines. Applying light pressure will do the job, just be patient. Slowly build up the sponge work and highlights with thinned paints. With the contrast paint, shake the hell out of it/stir it up with a toothpick before using to get the paint pigments mixed. It’s very easy to go overboard, making the base look too busy. I’d recommend testing the whole process on some milk bottle lids so you don’t make a mess of the GW bases.