Once you have 2 approved thumbnails, begin gathering the needed reference for your environment designs. These can include 2D and 3D references.
3D models for perspective elements like buildings, etc. 3D models can also be good for lighting reference so your perspective or 3D elements match the lighting in your scene.
Sketchup's 3D Warehouse is great for free Sketchup models for perspective reference.
Specific material references like trees, foliage, mountains, rocks, etc.
Photo textures to add depth and realism to your surfaces.
Images found on Google.
Textures.com is great for high resolution photo textures. 15 free credits per day. You just need to sign up with your email address.
Susan Bonner - Concept Art - Landscape - Using Sketchup to bring a PNG into PSD Part 1
Part 2 - Sketchup for Concept Art Rotating Styles - Part 2
Atmospheric Perspective Properties - color, space, value and line
SketchUp - Atmospheric Sketching - How to Use Fog to create space
SketchUp- Landscape Painting - full guide
Using Sketchup to build a previs of a spaceship attacking a Frank Lloyd Wright House - Susan Bonner
Pagoda Tutorial - Digital Imaging II - Fog and Color in Sketchup - Susan Bonner
Pagoda Tutorial - Digital Imaging II - Fog and Color in Sketchup - then to PSD to desaturate so that we can draw on it. - Susan Bonner
Exploring views in Sketchup using the camera and depth of view. - Susan Bonner
Timothee Meyrieux :Feng Zhu Style - Reproducing Howl's Castle using Sketchup
21:00 moving into PSD
Using Architecture Methods for Concept Art is a good idea - notice the use of Photo Compositing in addition to using the 3D Warehouse for time savings.
Designing Uncharted 2 with Sketchup as a design tool
Cool to be able to 3Dmodel in sketchup too - but use your tool box KCADers which has Modo or Maya to do your modeling to this extreme.
In Sketchup I recommend that primarily use the 3D Warehouse.
Nolan Nasser - Library for the Game Of Fate of the Elder Gods by Greater than Games and Dice Hate Me
Nolan used Sketchup, Compositing and Digital Painting to get at his final. While Compositing he used the Color Match technique under image adjustments to make the book photo layer match the color of the rest of the image. Nolan works primarily on one layer after all of the compositing. Dutch Tilting the Camera as the last step was very effective in giving a strange eerie mood. See his work at n3art.com.