Start by painting the structure of the mountain. Using a flat paintbrush, paint the profile or shape of the mountain. This layer of paint is actually an underpainting, to be built upon as you develop the piece. This means the paint should be applied thinly and quickly, without regard to detail. Thin this layer of paint by first dipping the paintbrush in turpentine or paint thinner. You cannot thin oil paint with water.
You may wish to allow the canvas to dry before trying to paint more of the mountain. Oil paint can take weeks to dry, but if you apply only a thin layer (as you did when you painted the structure of the mountain), it will take only a few hours or a day to dry. Depending on how wet the canvas is after you've established the underpainting, the details may become murky if you try to paint them without waiting a while. Switch to a smaller paintbrush to start painting details. The smaller the details, the smaller the paintbrush. Details on a mountain may include snow, trees or brush. Look at a photograph of a mountain for ideas; notice that some mountains have a "tree line" -- a point beyond which no trees will grow. Often snow-capped mountains will have a point below which there is no snow. Photographs are useful for deciding what details are realistic and what make sense.
Logic tells us that objects that appear in the foreground will be large and will diminish in size as they recede into the background. Mountains, although they are very large, will actually measure small on the picture plane simply to maintain the illusion of distance. In addition, objects become light or faded in color the farther away they are from the foreground. To give an appearance of depth in your piece, mix the paint that you use on the mountain with a small amount of gray.
The approximate color of the mountain should match with the type of landscape you are painting. A mountain set in a desert landscape will often be brown, and a mountain set in a lush forest landscape will be green or gray in color.