Others will have different opinions, but my suggestion is to use a 3/4" or 1" foam board from Lowes or HD on top of the plywood as a base for the layout. You can glue it to the plywood with construction adhesive. I then mark on the foam where I want my roads to go and cover them with duct tape. I then paint the entire foam base an earth brown latex color and, while the paint is still wet, I sprinkle green and yellow scenery ground turf until I get the desired mix I'm looking for. After it dries, I remove the tape and paint the roads either silver cement color or black and add road stripes with a white paint pen. That gives me the base I am looking for.
At that point, I start adding buildings and structures, signs, people, telephone poles, etc. and figuring out where to put tunnels , mountains, trees, foliage, etc.
Depending on the size of your layout, you should probably pre-plan as to where you want different areas to go such as industries, towns, sidings, farms, etc.
There are lots of threads on here in the layout forum that should also give you some ideas and inspiration.