How big do you want your mountains to be? If you want them to be fairly small e.g. for a small Christmas garden display, you could make a cardboard cone and start laying on glued strips of paper. Keep adding and smushing the glued strips until you have achieved a mountain shape. OTOH if you want a really big mountain, that would require an infrastructure of wire mesh. But the procedure would be the same. Just keep layering glued paper until your structure starts looking like a mountain. Good luck with your project!