Best way to get used to painting backgrounds is to try and imitate someone else's work. Preferably someone who knows what they're doing. Take a look at the backgrounds in some of the scenes for FiM, especially the outside ones since they tend to have less details. Note the shapes and lines they use to create the background, and how simple they tend to be.

Example:
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That mountain is really easy to do. Just a generic mountain shape, a few lines to suggest rocks and crevices, and a bit of snow to suggest platforms. Working with rocks is actually really good practice, because they're nothing but simple shapes.

Another Example:
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I went and did this scene myself not too long ago. Take a good look at those rocks. How many colours do you see? 3, maybe 4 if you want to include the small bits really far back. Just by using a lot of straight edges and 3 colours you can create a nice rocky cliff, and it really doesn't take long to make something like that.