Try watching The Joy of Painting, it still reruns on PBS in many places, even now that Bob Ross has died. If your local library is anything like mine they may even have copies on video.
He likes to do mountians and every seasion he does a fall painting or two.
Long ago I did a freelanced NH layout in my basement featuring the fall New England rolling hills. It was a rental apartment so I went to the butcher shop and got about 50 feet of white butcher's paper that I taped to the walls. I then painted the sky using a broad brush and tempera paints. I started at the bottom with a very light blue and continually added more blue as I worked my way up ending in a dark sky blue. It was streaky and it looked really good.
Living in Connecticut at the time, I was able to take photos of the rolling hills that I wanted for back ground. I painted them in various stages, The most distance were a very pale greyish green, each hill got darker green as they were closer. When dry, I used sponges dipped in varying colors of green and ellows, oranges and some reds and dabbed them all over the nearest hills and then faded the paints and did the next ones back. I left the last ones alone.
Basing the painting on the real world provided a very attractive back ground that got lots of praise from visitors.
Fall is only a few months away so get your camera ready and go shoot those hills.