Once crushed, they are very floppy. When most of the more vertical strips are in place, I add horizontal bracing strips, also about every six inches. This is where the hot glue gun really comes in handy. When finished you will have a lattice work of cardboard strips with six inch gaps and the general outline of the finished mountain. To the cardboard must be attached a layer of plaster impregnated sheeting that is strong enough to support the weight of the plaster castings to come later.
Most articles describe the use of plaster soaked paper towels.
I have never tried this method, but except for being very messy, I am sure it works well. I think plaster casting material is much easier to work with. The kind I buy is four inches in width. Woodland Scenics also offers this product. After placing the drop cloths in place and the bucket of water at a convenient height, cut several strips of plaster bandage from the roll, about one to two feet in length.