Getting some texture
Now that the everything is set in place you can apply filler/plaster to give everything texture, cover over mistakes and provide extra adhesion. When buying your filler/plaster try to go for the bigger, cheaper wood fillers. They tend to be much stronger than plaster and once dry go quite hard, not to mention that filler tends to take on shapes easier than plaster due to it's thicker consistency. You'll also use quite a lot of it - for three boards (the three shown at the end of this tutorial) I used an entire tube, which cost about $12.
Anyway, to apply the filler just use pieces of thick, stiff cut cardboard (I cut mine around 150 x 90mm [5" x 3"]). You'll need plenty of applicators so cut a few for each board you're going to do. Squeeze out of the tube and straight on to the applicator and then use the applicator like a spatula - work the filler into the board and the cardboard, filling every crack. This will make it stick all the better and add further strength to each structure. Once you've done this you can use an applicator, or anything for that matter, to give texture to the surface. You could stipple the surface, drag lightly along it, stab it with skewers, mould it into shapes - the choice is yours.
NOTE: Be careful not to make the filler structural or too thick, use cardboard and other items to build shapes and then use the filler to give it texture. Try not to make filler/plaster more than 8mm (1/3") thick.
WARNING: If you've used polystyrene ('foam') to make any of this terrain be sure to make sure that it's completely covered in filler. Spray paints usually eat/melt this foam but as long as it has a layer of something over it it'll be fine. You were warned!
Here is how the board looked after I'd applied some filler. It's great stuff to use and dries hard, leaving a good surface to play on. After applying the filler leave it overnight to dry, you want to make sure that it's fully set before you work with it.
To get this effect on the filler I dragged an applicator along the surface, giving a flat surface with gaps between that look like eroded stone. You can apply all manner of different textures using household items (sponges, skewers, spatulas, modelling tools and anything with an interesting texture are all recommended) and the filler usually cleans off pretty easily with some water.
You can see that I've also smeared random filler across parts of the board. This helps to give some extra texture to the board and helps it to look more 3D, rather than just being flat. You can use cardboard to put up gentle hills and slightly raised ground, then use filler to make it smoother and more realistic later aswell, if you like.
Once the filler has set you can add an extra layer of filler if you need to, again make sure that you work it in to the surface so that it sticks well and don't make it too thick. It's better to have fairly thin layers and use more cardboard (as it's free, light and should form the bulk of any structure/hill/raised structure).
Another thing you might like to do is paint parts of the board with PVA and then pour sand on them. This is another great way to give the board/terrain items texture and is super cheap (go to the beach and take a small plastic container, simple). Something else you can do is mix different grades (re: size and texture) of sand/gravel together and pour them onto the glue at the same time - this makes for a more random sort of texture (I'll show you this method in a future tutorial).
When you're ready you can move on to Part 4, Adding some colour.