Ther are many scenery books that deal with building mountains.
"A Realistic HO Layout For Beginners" and "The Pennsy Middle Division in HO Scale" published by Kalmbach Publishing Co. deals with cardboard webbing to produce your basic form and then adding plaster cloth. I think Woodland Scenics also has a book on building mountains and how to use their products.
I have always used the plaster and wire screen method in the past, but I am using 2" foam board now on my present layout.
I like it, it's faster, and you can really see what your final form or shape will look like.
Hope this helps.
Marty