No problem, it's actually as simple as possible:
1. layer of sand
2. diluted wash on whole sand area: I use vallejo burnt umber (that's the name iirc, delicate brown wash similar to devlan mud) but diluted smoke or brown will also do the trick, add some olive if you want base look bit more "fresh"
3. when dry (and I mean it - when it's totally dry) - aged bone drybrush (or any middle bone color)
4. delicate white drybrush
5. as option - you can sprinkle some chalk / pigment to make base look kinda dusty
6. matt varnish
Voila
