Basically, I glue small pieces of cork to make the stones on which my models stand, and I cover the remaining plastic with thickening medium to add some relief to the base.Once the medium has hardened, I use an old brush to apply water-diluted PVA glue on the places where moss or grass is supposed to grow and I sprinkle the whole base with fine sand.
When the glue has dried, I basecoat the base with Chaos black... and the painting job can start! For the rocks, I apply a base of dark grey, such as GW Adeptus Battlegrey. For the ground, I use dark brown, like Vallejo Chocolate Brown and for the grass, Vallejo German Cam. Dark Green.
I then drybrush the stones with lighter and lighter colours (Codex Grey, Fortress Grey and Bleached Bone) and paint the sand with different tones of green. Et voilà ! Two bases (rather) quickly done.