A large lump of polystyrene (the bigger the better - I cheated and did one with a flat back to put on the edge of the table)
Some foam core or MDF off-cuts
A little balsa wood
And some paints (to fit in with your terrain)
Objective 1: Grab a Carving Knife
To make a rough rocky outcrop out of a perfectly oblong piece of polystyrene is not something you can do with surgical precision. Raid the kitchen and get the biggest, roughest (but preferably sharp) kitchen knife. A bread knife is ideal.
This is going to be messy. I would STRONGLY recommend doing this into a bin bag. Polystyrene has the habit of disintegrating into thousands of tiny pieces that want to adhere to every surface. I made some rough cut marks with a felt tip pen to work out the slope of the outcrop and then started sawing into the stuff.
Periodically knock off all the polystyrene balls and see if you are happy. You want lumps and bumps, it can be pretty sheer, as I intended to cut out a series of steps up to the fort at the top.
Once you are happy with the overall shape of the "rock" you need to turn you attention to the steps.
Objective 2: Cutting out the steps
This needs a bit more thought and precision, but you still need a pretty vicious weapon to achieve it. A long bladed craft knife is ideal. Ideally don't cut the steps into a sheer face, you want a bit of a slope to work from. Mark out the graduation of the steps, you want them to be about a centimetre apart. Cut the first one out, then dig in deeper with the second and so on. You want nice squared off steps, I went for a width that would take a 2p coin.
Objective 3: No place like home
For the fort at the top, you need to use some foamcore or MDF for the walling. I went for a simple design with a single doorway made from balsa. You can be pretty rough with the cuts and in fact I deliberately made the fort look like it had been shot at before. About a half inch down on the inside of the fort stick a rough walkway and finish the whole thing off with a simple cocktail stick ladder and then push some cocktail sticks into the front of the fort.
Objective 4: Rough texturing
I like to use powder wall filler for a lot of the texturing jobs. I can make it to the consistency I need for each job. You are after a double cream consistency. Slap the wall filler all over the rock first. For added texture you can sprinkle some sand and gravel on it while it is still wet. Apply the creamy filler with a big old brush. This job will kill the brush, so don't use anything special. For the fort you want clotted cream consistency, or perhaps more appropriately Humous consistency. Apply this with a dining knife, you want a pretty blunt one as you are going to be plastering rather than slopping on the filler.
Objective 5: Undercoating
I use a chocolate dark brown as the base colour for all my terrain, so to be consistent use whatever colour is your base colour. I figures that black would be too stark. Thoroughly coat the whole piece in your base colour and leave to dry. This will fix all of the texturing and give you a good key for the next stages. Make sure you work the paint into the crevices, you want to obliterate any sign of the polystyrene.
Once this is dry, although I don't recommend it, you will find that that whole thing is pretty robust. All of the polystyrene has been sealed with the filler and then by an solid undercoat.
Objective 6 Finishing off
I used a mid Grey as my main top coat and you'll also need some white. Pick out the bits of the rock that look like rock. I figured that the flatter areas might have some vegetation, so I was happy not to cover these bits in grey. For the grey parts you are looking for a solid coat which hides the brown underneath. You can give the whole thing a heavy dry brush with the grey to give it more consistency if you like. Alternatively if you can be bothered, give the brown a caramel dry brush.
Once done, give the fort a very heavy white dry brush. You want a bit of the brown showing here and there in the cracks and the texture, but not much.
Now use the white to give the whole rocky part of the piece a light dry brush. This will even everything out and give you a nice overall consistency in colour.
All I did then was to retouch the cocktail sticks on the fort back to brown (and gave them a caramel highlight), I blacked out the door to the fort (you might prefer having an open gateway or a wooden one). The last job was to add some splodges of PVA here and there and then stick on some clump foliage and/or static grass.
Here's mine, no masterpiece!
I'll post up some shots of the fort in action soon.