The best way to create the mountains structure is to use 3cm polystyrene sheets foam. Put the first sheet making it lean on the external frame of the bearing structure and on its horizontal reinforcement spacers. Make the relief gluing togheter different sheets, paying attention to reduce the size going up ( imagine a pyramid, but in our case with irregulare sides), in the way to create the profile of the montain with the desired slope. Create some level curves, as you see on a map. After having carried out the desired profile, cut with a xato knife the corners of the sheet to combine the upper sheet with the one below. Don't worry to be too accurate in this stage. Now you have to cover this main frame with some papier-mâché to make the slope more realistic. The papier-mâché is made of choping page of newspaper and putting them in the hot water for some hours. Now you have to mix the material to make a mush. Leave it rest for some hours (good for the due is a paint mixer mounted on a drill ). Now you have to take off from the mush the water in excess, using a duster as a filter. At the end you have a mush not so wet, but easely mouldable. Add vinyl glue and glue for wallpaper, until the mush become slippery and stickly. You can now use the papier-mâché as a dough: put step by step small part of it on the polystyrene used for the profile, in the way to create the slope. After drying (from some day to some weeks, depending on the water in the dough) paint the papier-mâché with a background color.(green if it will be a meadow, gray if it will be a rock).