the bases for along time I use a sand fine and hard gravel mix applied using PVA glue let is set then paint with wilco match pots coffie test pot there a £1 now i think, let it dry hard then dry brush with a cream to high light light sand color will also do the same job, , then dot some PVa and static grass, I bought a huge bag of Kalistrra . the edge of the base line if wanted a run a spearshaft round to finish it off. I do apply some static grass clusters if needed but find the one static grass is good.
The Wilkos paint test pots will go on any thing like buildngs or MDF amd on sand bases work fine