Before you even start worrying about nice angles, learn to make your characters part of the backgrounds you draw.
Using green for the light and red and black for shadows... you see, to make a good background, you need a light source. You then need to consider how that light source will effect the background. It doesn't get lighter towards the mountains, it should be dark there. He should be casting a shadow. I assume the lense flare was to create a light source, so in front of the slime monster, there should be shadowing and highlighting around him.
The slime is awesome. I can't draw slimes! Or monsters. But I agree, your background is pathetic. And that's because it looks like mountains pasted on a cloud render pasted on a gradient with a lense flare dropped on top. Am I wrong? Let's start with those mountains. The render has... some logical shadowing... some not. They look like the light is coming from in front to our left. ...The light source is behind them. The mountains look pasted on. You may find that your newest friend is Mr. Photoshop Grass And Or Foliage Brushes! I have some grass brushes with an image pack for free use if you're interested in trying to play with that sort of thing. Here is my grass.
http://calthyechild.deviantart.com/art/Grass-brushes-image-pack-92536494
Now, you see, the glory of these collections of various lines is that they let you paint grass fast. They let you disguise building lines and things like those mountain feet. They let you build up foliage. You can get a TON of grass and foliage brushes if you use Photoshop.
There's no simple step. I paint elaborate backgrounds for my comic. The cue is that--paint. It looks like you're Photoshopping it up. Your slime is pencilled. Try pencilling your backgrounds and coloring them too. Copy from real life. I've drawn my fair share of building interiors for practice and I still can't do perspective to save my life.
The ability to do perspective does not change the ability to do a background. People complained about the one I drew perspective lines to get the math right for because it looked wrong. (shrugs) Maybe I did do it wrong. I can't think that way. But I've never drawn perspective lines for forests. You just have to put things everywhere. It's all about chaos. I suggest you move from doing mountains and challenge yourself. Indoors, or in a forest perhaps.
Treat backgrounds like they're real. Study stock photography. Sxc.hu is my most visited site these days, or one of them. o_O There's no magic step. Study them and you'll get it. You already did a cool monster. Now learn to make that monster interact with his background.
You don't need overly detailed backgrounds. You just need logical shadows and lighting to make your characters interact with that background. Backgrounds are there to be ignored, usually.
Which is why you don't always need a lot of detail. Here's one comic page I did...
Now, this is one of my simple backgrounds. The layers of snow and fog provide foreground interest without being detracting. Branches crossing over can do that too, and provide depth. The background itself is really just some blurry lines with a layer of a few evergreen branch brushes and some snow. But their feet interact with the snow, so they're not floating on that background. I wanted the attention on the characters there--not on anything else. An overly complicated background will distract just as much as a bad one.
I suggest studying Disney backgrounds and other animation backgrounds, to get an idea of how characters interact and how backgrounds are detailed without being too complex.
http://animationbackgrounds.blogspot.com/
Seriously, there's no trick to it, just get out there and draw things. Mountains are good to be able to draw, anyway. ;)