The next step is to cover the entire base with wood filler. I thin it down a bit with water to make it easier to apply. The thinner it is the smoother the final finish. You do not want it too smooth as the final effect looks much better if the ground has a number of irregularities. In this photo the filler is still wet.
Once the filler has dried I apply a few dabs of texture to roughen up the ground. The best way to do this is to mix fine sand, water, white glue and a bit of sand or khaki coloured acrylic paint. Alternatively you can use a ready made material such as Basetex.
Here you can also see a good recycling use for an old rail card
I then do the same using a similar mix but with green rather than sand paint. I only apply a few dabs here and there. Afterwards I usually add a few, very small, pieces of gravel, with white glue.
This step and the previous are not really essential but I find it adds a bit extra to the final look of the base
Once all the added texture is thoroughly dry I wash the entire base with raw umber with a fairly large but soft brush. I use a slightly thicker wash than I do on the figures as I actually want to stain the light coloured wood filler rather than just bringing out the details. If you have ever stained a garden fence or wood panelling you will know what I mean.