Glue some small pieces of slate (flat side up) to the base with super glue. Could add other bits as well such as skulls, weapons, etc.
Black - Prime at 15-20PSI
Mechanicus Standard Grey - Basecoat
Apply white glue around the rocks and on the base while leaving a couple of areas without glue
Dip the base in sand. Tap any excess sand off and ensure the rim is cleaned up. Allow to completely dry (>1hr)
Agrellan Earth - Apply texture to areas without sand ensuring you slightly overlap the sand around the edges with a thin coat. Also, apply on top of the slate. The thicker you apply the texture, the bigger the cracks will be. Allow to completely dry overnight.
Baneblade Brown - Paint over entire base
Mechanicus Standard Grey - Paint areas of rocks that do not have the texture on them
Agrax Earthshade Athonian Camoshade Reikland Fleshshade - Apply Agrax Earthshade directly to base, as you're applying switch to Athonian Camoshade and apply directly to base while overlapping and mixing slightly with the previous shade. Perform the the same technique with Reikland Fleshshade .
Karak Stone - Lightly drybrush over entire base
Screaming Skull - Very lightly drybrush over entire base
Steel Legion Drab - Paint rim of base
Matt Varnish Airbrush Thinner [1-1] - Spray varnish entire model at 15PSI. Don't allow it to pool. May require 2x coats. Could also very easily brush varnish on.
Optionally add some grass tufts to base, best to stay in the brownish colour range.