I have learned a lot over the years on terrain building from several sources, not least of which was Big Gav on the now defunct FOW Forum. You can find his hill building tutorial here. Though the blog is long dead, it has some very useful articles and images: Gav is found more often these days on the Gold Coast Gaming page on FB. Which may be defunct soon, too...
I was asked to build some hills for a local gamer, so I've documented the process. First off, using a plain sheet of 6mm MDF, I measure the bases for the hills and try to make them different shapes and sizes and then try to fit them to the limits of the available material. I cut the MDF to shape, then trace the same outline on some high-density 35mm polystyrene, and cut that out to match the base.
When done, I use polystytrene glue (or cornice glue) to glue the poly piece to the MDF piece. You can also use something like acrylic silicon for this (aka "caulk"). Leave to dry overnight.
Then start shaping with whatever available: I use a combination of a serrated-edge steak knife, rotary sander and a multi-tool with wedge and sanding pad attachments. Power tools are you friend and they save a great deal of time and effort, but I have done hills just by hand before using the steak knife to saw through excess poly and then sand by hand.
|Roughly shaped pieces on the left, those still to do on the right.
I remove the majority of polystyrene where I don't want it, slowly carving the hill to shape and usually leaving a crest line. I then sand the MDF to match the required slope, finishing the shaping of the hill.
|All pieces sanded: you can see some of the crest lines quite easily.
Painting is simple: I paint the sand with a slightly watered-down dark or medium brown, followed by a lighter brown and then a highlight colour. Here the colours used were German Camo Medium brown, Green Brown and Iraqi Sand...I get the first two mixed in cheaper PVA to match the Vallejo colour. The Iraqi Sand dry-brush is done with actual VMC Iraqi Sand as it covers really well which saves time and costs very little. About eight drops did all ten hills I was working on.
|Initial test grassing...need to hold them somewhere.
The grass is applied on two stages (since I need somewhere to hold onto the hills): first application, dry overnight, then complete the process. With that done, these hills were complete.
|Lots of diluted wood glue and tons of grass later: hills.
|I usually have a few FOW bases handy during initial shaping of the hills,
to check that the slopes are not too steep for bases.