Here's a background I drew for a game I was working on not too long ago. These steps aren't in the order in which I did them, it's just to give you an idea of how you should go about drawing these things.
First what you want to do is sketch out the general idea of what you want. I usually always paste this on the lowest layer and the highest layer so I can reference to and from it easily.
Then I added a backdrop.
Then I worked on the mountains and blurred them a bit when I was done to give the impression that they were out of focus because of how far in the background they are. I also drew the bottom of the lake so I can blend it with the water on the next picture.
Take not around the edges of the water where it's faded a bit so you can see the ground underneath it where it's shallow. When I was originally working on this I did not detail the grass, I only used the base color. Details of that magnitude come much much much later in the process.
Again, I'm adding objects to the scene to give it detail. Everything here is it's own symbol. I also cut out a lot of the area in the trees so they would overlap the fog.
Adding more fog.
Adding some more detail. This is usually around when I would start detailing the grass and ground. Also, I drew a lot of the symbols early on with their base colors and came back later to add detail. Especially with the large boulders at the camp. If you compare this picture from the one with the grass and ground around the camp, you'll see where the detail is placed only where the boulders won't be covering it up.
This is zoomed in once. This is generally how far I zoom in when I draw with brush. Sometimes I'll zoom in twice.
Even now, there's so much I would change or do differently if I were to redraw this background. Each one is somewhat of a learning experience. You only get better if you keep practicing and trying new things.
