You can use textured paints, polyfiller/spackle which is like a plaster or green stuff. GS is probably the simplest to use so I'd suggest going with that. Otherwise you could pick up some already textured bases, most I have seen are made of resin though I haven't used these as they don't match what I want.
It won't match up with every board but provided you have enough green on there to break up the stone, it will blend excellently with most things.
What I would do is a mix of well manicured path, long grass and dirt. I assume back in the old days someone would go along with a shovel and just clear the top of the path from grass and dirt and that would be it. Citadel guard or GotFC I would do cleaner paths on.
Anyway, Step 1 GS approximately half the base in the direction the warrior is running/looking.
Step 2: Add a small amount of sand/dirt in selective places on the side of the path and maybe 1 or 2 in the middle of the grass (on the edges of the base). Paint as desired - i would suggest black with a dark grey drybrush and a very light but brighter grey drybrush after that. Try to brush in the same direction. Having the grey too light will make the base stand out too much and won't blend with the table. Brown for the sand.
Step 3: Add grass to the remainder, blending some with the path. If you have more flock you can add it to break the grass up if it looks too uniform.
Here is a link which may be helpful. Some other articles at the bottom. The gravel they used is a bit big for my liking, but it gives you something to look at. I might try doing this and putting some pics up if i have time.
http://www.belloflostsouls.net/2010/01/hobby-making-cobblestone-bases.html
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