I started using this technique on my Romans and Ancient Britons
I wanted to make it as simple as possible because of the number of figures
I had a bottle of the Woodland scenic earth undercoat which I had never used. I mixed it with white glue and it worked better than putting paint in because the pigment seems stronger.
http://www.track-shack.com/acatalog/Woodland-Scenics-WC1229-Earth-Undercoat-Paint-Woodland-Scenics-WC1229.html?gclid=CIudvKe8rMQCFayWtAodoi4AOAAnyway, You need white pva glue and woodland scenics earth undercoat
A mix of sand and gravel. I use sharp sand, not builders sand. Builders sand is used in mortar and is too fine. Sharp sand is coarser and has bits of gravel in it. I add more gravel in various sizes from all sorts of sources but its important to not go overboard. (I do a lot of travelling and so take jars with me to put any interesting stone/gravel in, please feel free to insert dogging jokes)
Mix the white glue and earth undercoat. I don't have an exact mix ratio, but you want it to be free flowing. If it goes stringy you can add a drop of water, but don't add too much. we want the glue to be as strong as possible.
Slap it on the base
Push the whole lot through the sand/gravel mix. Shake gently, I tap the base against the side of the box. Fill in any bald patches by hand. If you want some larger bits on and none stuck, just drop some more glue/earth tone on, drop a piece of gravel on followed by a pinch of sand and leave.
Let the whole thing dry
Mix up the white glue and earth tone again, add water and mix until its a milk like constancy.
Get a fairly large brush and load it with mix, place it on the base, it will run off on its own. Completely cover the base.
This sounds like a pain in the arse, but it is surprisingly quick. Don't try and paint it on, just dip it in the mix and blob it on the base
Let it dry
Pt2 to follow