For environments, I'd start with a grid plane, get the general layout of what you want to build out, then start laying down rough textures for where you want everything to be. Like dirt for your roads, grass for your plains, rocks for your cliffs. It doesn't have to look pretty. It just needs to show you where you intend everything to eventually go. Then I'd start populating it with your foliage and rock meshes. Painting in your trees, grass, bushes, cliff faces, and whatnot. From there, it's all about refining. Bringing out the details, tweaking things to taste.
Think of it like building a model railroad.
For architectural work, I'd do a blockout with basic primitives to get the shape and scales down, and then start adding in the textures and details once that's done.