Get out some white cardboard, or buy some bristle board, and practice a couple or four techniques. Try to keep a log of each experiment so that you can duplicate your effort if one of them turns out half to your liking.
Find a light blue, hopefully in the "oops" section of a paint store (wrong mix), and paint a band the length of your backdrop from highest point to below the bench top...on the wall of course. Then mix some white paint of the same type into the batch and paint a swath partway down, parallel to the bench surface, and as it dries, try to feather the top edge into the darker blue first painted. Add more white, and repeat, but start lower. Do this a couple of times and you should have a decent gradient from darker to lighter blue, almost white at the horizon. You
can do it, but you must learn that you can do it, and the best way, once again, is to paint up a mock-up first.
For hills, start with a medium green, decant a bit of it and add two or three drops of blue to the smaller portion. Just free-hand some low mountain profiles, but they'll have to be visible in the distance, so they'll be highest on your backdrop. Mix em up...if you goof, you can paint over it. Then paint lower mountains in front of the first with the medium green. When that dries, add lower hills with a wee bit of yellow mixed into the original green.
For clouds, take a piece of thin stiff cardboard (cereal box?) and cut out a hole with irregular edges that you can easily put your hand through. Hold that stencil about three inches in front of the backdrop (well above the mountains please) and give short bursts of a white spray can held about 4-6" in front of the edges of the cut-out portion. Rotate the stencil to get a variety, and try super-imposing some of the shapes to creat larger clouds.
Practice first.
Here is my first ever backrop. It doesn't look nearly as good from 4 feet away, but it looks half decent in an image. Surprised the heck out of myself.