Someone asked this question a few months ago and I responded with this:
First, spread some white glue across the base (I have used a pop sickle stick and an old paintbrush, so whatever you have lying around is fine). Try not to get the glue on the feet of the model, but it isn't a huge deal if you do.
Second, dip the whole base in a tub of sand. Pull it out and shake the extra back into the tub. If you have sand on places you don't want, get it off now before the glue dries and makes it really hard. Let this dry for a while.
Third, paint. Normally you haven't painted the model yet so the sand gets a black basecoat with the model. Since you have painted the model already, paint the sand black. Give it a heavy drybrush of Scorched Brown, a medium drybrush of Graveyard Earth, and a light drybrush of Bleached Bone. Make sure to be neat around the feet and legs of the model or you will have to go back and touch it up.
Fourth, add static grass. Put some glue dabs on the base and then dip the base in a tub of static grass. Shake the extra grass off. Some people say you should blow on the grass to make it stand up but I have never found that to work. Let this dry.
Finally, paint the edge of the base with Graveyard Earth.
This sounds like a lot of work, but the hardest part is painting the sand black; everything else is easy and very much worth the effort. The bases are going to look like anything on the GW website.
The guy I suggested that to seemed to like it, so hopefully you will too.
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