Laying the Foundations
You can buy ruins and the like from game suppliers but they're usually only a couple of millimetres thick, expensive and need to be based and painted anyway. Using cardboard and old packaging (which is easy to find and plentiful) you can make almost anything for next to nothing. You're also doing the environment a favour by recycling materials rather than using new plastic ones, though adding bought items is easy and can add items of interest to your board.
Now that you've got your work area set up you can cut your cardboard to make any shape. The board made in this tutorial has a crumbled tower made from cardboard, gravel and an old terrain wall that I had lying around. There is also a low stone wall (made from gravel) and some reeds surrounded by rocks (made from some broom bristles and gravel). When you're putting together all these items make sure that you use plenty of PVA, make sure it fills all the cracks and give it a while to set in place (a couple of hours or so in a warm room will do). This ensures that everything sticks like proverbial and therefore, hopefully, nothing will fall off and your boards will survive a few knocks. Just make sure that there isn't a tonne of PVA in blobs (unless this is a deliberate measure, blobs and drips of PVA make for great blood/alien goo etc.), it's quite noticeable in the final product (unless you go over it with plaster/filler, but try to avoid having to rely on fixing mistakes by covering over them).
This is what my crumbled tower and rock wall looked like about 15 minutes after everything was in place.
Note liberal use of PVA. You'll find it'll stick better if you 'rub' the two pieces of cardboard together, ensuring an even and complete cover of glue. Also, try to glue rocks to more than one thing, they're heavier and the more glued contact points the stronger the bond will be. If you want to have any items of interest that look submerged (half a tank, a skeleton, half buried armour, pikes/spears etc.) now is a good time. If you want to make a wooden pole fence you can drill a few shallow holes for your 'fence posts' (I use bamboo skewers) then glue and slot them in, the same goes for pikes coming out of the ground (having a shallow hole [4-5mm or 1/5"] also helps them to stay in play far better than if you only glue them to the surface).
These are the reeds surrounded by rocks. Just make sure you use an old broom/brush for this as cutting into your wife/girlfriend/Mum/Dad/housemate's new broom could be physically perilous.
The layout of the board.
Some time later...
Now that all the glue is dry this is what it looks like. Once it's all dry you can move on to applying filler/plaster.
After a couple of hours you can commence with Part 3, Getting some texture.