First up grab your materials:
I seem to have missed a photo for this step but after everything is dry base coat your stack in black for anything that's stone and brown for your grassy areas, I use these acrylic paints:
but any craft paint will do.
I just used what I had to hand which was Codex Grey, Fortress Grey and White Scar.
Once your stone work is dry take some washes (I use green, sepia and black) and start added patches to break up the grey stone, again don't worry about how the ink goes on just slap it on.
Next you can finish of the grass base by highlighting up through the green's to yellow on the areas that you under-coated brown.
Now start pasting your flock onto your ruin using a coffee stirrer, try to stick to edges or anywhere that you could see leaves fall settle and pile up.
Next mix up your green moss flock in a similar fashion to the previous step and start piling it on in any cracks or crevices.
I add a few little bits of the home-made leaf litter to give it a little more texture and for the final touch I add a few bits of the door-matt with some tweezers and a tiny bit of super glue: