The desert tiles were 'painted' with PVA and then sprinkled liberally with fine sand. Real sand that is - I bought a bag from a DIY store. When dry, small random dabs of PVA were applied and sprinkled with 'grass' flock. The whole lot was then treated to a matt spray varnish.
For the 'snow' tiles I used a similar process but with Woodland Scenics white flock. First I sprayed the bare tiles with a rough coat of matt white - this gives the finished product a slightly patchy effect, which at least to me, looks better than a neat pure white finish.