Simply create a basic structure with polystyrene, celcon block or expanding foam. Then create a natural looking surface by applying ProRep Terrascaping Texturing Compound, and model to look like rocks or wood.
Finally waterproof it with ProRep Terrascaping Sealing Resin (sold separately), coloured with your choice of ProRep Terrascaping Resin Colour Pigment (sold separately) . The resin can be left bare or have bark, coconut fibre or sand pressed into it to make a rough, naturalistic surface which allows moss and plants to grow up it.
This powdered modelling compound is used for the second stage. The power is mixed with water to make a paste and then spread over the basic structure to make a textured, realistic base ready for sealing.