Start with a Citadel 25mm round base. It might be a good idea to make a batch of three or four "test bases" at once for efficiency's sake.
Cut around 10 Centimetres copper wire. This will be used to manipulate the textured paint.
Bend the wire upon itself. You'll need to do this or it won't be strong enough to work with the texture.
Use the pliers to twist the wire.
The result should be something like this. Such a tool will be strong enough for our purposes.
Use the wire to get a glob of Citadel Textured Paint. For this tutorial, I'll be using Stirland Mud.
This should be enough to get started.
Start spreading the texture onto the base.
If you're using a slottabase, start by filling the gap where the miniature would fit.
Try not to make it smooth. It will work better if the base has a rough finish.
Keep going. Cover the whole base with the stuff. If you so desire, you could use the textured paint to base your miniatures, although I usually stick to using sand.
Once the base is covered, let it dry. It should take several hours. Once dry, the Citadel Texture will be hard enough for you to start applying paint to it.
Once it's dry, you can start to test out different colour schemes. Here are just a few of the test bases I've made. Check my blog for more ideas on how to base your army.