It's real easy, you drape a piece of wetted gauze over the painted and weathered model. Have it positioned the way you want it then before it dries out, load a small brush with some diluted PVA glue. You may choose also to add some acrylic paint at this time as it will soak into the gauze to ease with painting later. Otherwise wait till it dries before you paint. You can also sprinkle some model railway grass pieces depending on the type of camo you wish to have. Hope this helps. :P