There are several methods adding ground cover. If summer, spring or fall is desired, use ground foam (by name, Woodland Scenics makes quite a variety) in its various colors. Sprinkle it on, and mist it down with a 1 to 1 mixture of white glue and water, with a drop or two of liquid dish washing soap to break the surface tension of the water so the mixture will flow. Ground foam can get to be expensive, so try using sand and dirt found near your house for a substitute. The best effect comes when you layer on varities of ground foam.
Many railroaders have another method designed. They paint the layout with a tan colored paint, thinned, as a base coat. Then, the foam is sprinkled on while all this is still wet. The paint holds the ground foam in, and the glue is sprayed on as above to keep it secured. If you don't like the glue, you can use matte medium- mix one part of it to 3 or 6 parts water, plus a drop or 3 of dishwashing detergent. Many modelers have had success simply misting on rubbing alcohol.
If winter is desired, use either baking soda or flour as the snow. Glue it down the same way you did the foam. Some people like to use white marble dust, sold in hobby shops for this purpose. Others use dry plaster- they sprinkle it on through a sifter. Then, to keep it in place, water is misted on. If you try the paint method abovem try white instead of tan.
Now for ballast, the crushed rock the prototype puts between the ties to allow for drainage, and to hold the ties in position. Use a small spoon to shovel it on, grade it smooth with your fingers and use an eyedropper to apply the same solution as above. Before gluing, be sure the rocks don't interfere with the trains, and that turnout points still move freely. For another explanation, see track work.
Trees, bushes, and babbling brooks
Trees are always helpful on a train layout, and I make mine by
finding twigs outside and gluing them in holes I drill into my plywood
surface. You can also glue lichen (pronounced "like - N") to the twig,
to give it leaves. Many companies make premade and tree kits.
John of Rome, GA. suggests that you pick golden-rod weed in the Fall after it's been out for a few days but while it still has the seed pod. Cut them to size and spray paint green for HO and N scale.
Lichen makes great bushes, too. Buy a bag, cut it up and glue it down. Smaller to medium sized cuts model bushes well. You can also cut sponges, paint and glue them down for a cost effective substitute.
As far as water goes, I recommend using an epoxy-resin. These come in bags of tiny cubes that melt when heated in a pot and can be poured into place. They usually dry clear.
Rocks and the like
As for large rocks, use rubber molds. Fill the mold with plaster, let it dry then break out the rock. Paint it brown-grey with water colors. Several molds can be combined to make a rock cliff, and you can fill the gaps between the pieces with extra plaster. Paint it in the same manner.
As for small rocks, just step outside. Almost any piece of stone will work. Glue it down with white glue.
You may wish to model a fill or cut. To do this, you can use the method above for making a hill, but leave a side sheer as a cliff (for the cut). And for a fill, you could make a bridge or something like a pier to bring an area up to level. Maybe some "concrete" forms on the side of the roadbed could model a retaining wall. The best method to design scenery is to look outside; look around you. Anything you see you can mimic on your layout. The methods above barely scratch the surface of what model railroad scenery is all about.