Carve out a basic hill and roughly shape it to your liking. Use a marker to draw the approximate shape of your cave. Keep in mind that this is just a rough estimate of the shape.

Now, it is time to gouge out the cave itself. Star off using a hobby knife and cut out as much as you cutting in a crisscross pattern. Pick out the pieces by hand and start digging deep into the hill with something like a screwdriver or even your nails. Be very, very mindful of the room you have to work with. If you are not careful you may dig right through the bottom of the foam or even the top of the hill! So use your best judgment and work slowly and steadily.

Once you've gouged out the cave, it is time to add some texture. Place some coarse gravel and pebbles at the mouth of the cave as it will break up the monotony and make it a bit more foreboding. We chose to texture the edges of the cave to represent the rough exterior of the cave.

Finally, paint the cave. We used lots of browns, but greys could do very well here as well. The addition of static grass completes the cave and makes the piece look a bit more natural.

To the right is another example of a cave. This one was made by stacking chunks of roughly shaped foam upon one another, gluing them together, and texturing the areas where they meet. It is a stand alone piece that does not go into a hill, suggesting that the cave descends into unknown depths. It was painted to be a bit more blasted and horrid, you can easily imagine running into this kind of terrain in the Chaos Wastes!