I have used many techniques over the years.
back in the dark ages I used metal screen wire over wooden props covered with plaster.
Later I followed the hardshell craze with plaster soaked paper towels over crumpled newspaper forms to establish the basic shape. I then added plaster on top of that that I either carved to rock forms or used molds.
Later I started using strips of cardboard to weave into wicker work shapes establishing the land mass shape, and plaster in paper towels would cover that, and the rock areas would get extra plaster to carve rocks into, or again I'd use molds.
For a while I was using this stuff called fabudrape that I was getting at craft stores. it was fabric that was soaked in a water soluble glue. you could soak it in water, and it would get all soft, and you could lay it on the cardboard forms, and it would harden when it dried. I would add plaster for carving rocks, or moulded rocks where I wanted rocks, and everywhere else I would paint the fabric with acrylic paint, add add ground cover directly to it.
Now more often than not I make my shapes with the cardboard strips, and soak handiwipes in white glue, and apply it to the card board wicker work. If the area is going to represent soil I'll paint it with acrylic paint, and then add ground cover. again if it will be rock I will either add paster to carve or shape with molds, paiting the rock shapes with acrylic paints when they are ready.
I have also taken to cutting out boulders from dense Styrofoam, painting them with acrylic paint either before or after they have been fastened to the layout.
experiment with different techniques, as there are advantages and disadvantages to each. I now only use plaster (preferably hydrocal or house plaster- plaster of Paris is too soft, and is not durable. I have also used Durahm's Water putty in lieu of hydrocal- it is very hard and carves well as it is drying.) where I am representing rock, as plaster is messy. If making a portable RR avoid plaster it is heavy, and will crack with the stress of handling.
My railroad and scenery skills are visible in the logging mining and Industrial section in the thread Logging in Eastern TN in 1928 on the DG CC & W RR . I have been doing model railroad scenery for 39 years.
Bill Nelson