I just have the Mountains on a layer inside the open file I am working with. and clone it from there. If they are in my way I just push them away in another corner I dont need. Like in my sample left the elements, right the Mountain clusters.

If you work from a scan, as i do mostly, you should try to turn all mountains in one swap to a big brush, than brush them to an empty layer, like this you have a clean drawing layer without any white and can start my tutorial, its much faster than trying to erase all white surrounding the drawing.

Have fun.