1 - Thumbnails will make it better! I didn't like my first one, so I moved to the next, and then another. These take about a minute each, though I do think about them a little before putting paintbrush to paper... digitally. As you can see, quite messy but they get the idea across. I liked the third one best, as it had the strongest imagery and composition.
2 - Beginning with the background, I use a few layers to build up the furthest scenery. I liked the eerie yellow light, as it was beautiful but abnormal, perfect for this wasted landscape! I use a lot of lasso tool, since it works perfectly for these jagged cliffs.
3 - Adding more elements, I make sure that the yellowish light has something to work against, so I move toward blues in the shadows, especially in the foreground.
4 - Constantly referring back to my thumb to make sure I keep a high contrast, I add the bases for the front. Keeping the size of the rocks in a realistic ratio to those in the distance is quite important, though I'm not sure I pulled it off here! Also worked on the water here.
5 - I start detailing, flip the image horizontally to see and correct any composition errors, then double-check my colors and distance. If something is too blue, or the cliffs are too dark, now is a good time to fix that, rather than later when you've spent ages on detail.
6 - Well, time to add the train! Oh wait, it looks weird, and unnatural. Also, it seems to destroy the focus, which is the waterfall and hounds. Not working? Get rid of it, if it's not vitally important to the painting. Also, I checked with another artist, and he agreed. Always ask passersby, family members, whoever, what they think of your painting. Offer them two choices (turn layers on and off), and they'll usually help you in some way.
7 - Add the last bit of paint to the rocks, add some subtle light effects with gaussian blur (around the dogs), and then bring it to a close with levels adjustment (as you can see in the finished piece).