Now that the scenery has been pre-soaked, it's time to glue. There are two good options for this step: white glue (diluted) and matte-medium. White glue can be thinned to about 50% strength with water, and it's widely available and cheap. Matte medium can be used as it comes and can be found at any art supply / craft store. Matte medium has the advantage of drying with a more flat final finish. Many model manufacturers market their own scenic glues - most are really one of the above with a different label.
You can apply the glue with a spray bottle or dribble it on with a pipette or the glue bottle itself. Be sure to cover the area completely. Since the glue is thinned, it takes more volume to achieve the same results. Thicker piles may take a lot of glue. too little and you end up with a crusty shell that is easy to crack. If the glue is not soaking all the way in, spray on a little more alcohol.
Although you may see a little color shift in the ground cover, the glue itself will dry clear - no matter how much you add. Allow the glue to dry before adding any additional layers or details.
That's it! Once you get the routine down, you'll be able to transform your layout in no time. You can use these same techniques to add ballast to your track as well. When you're finished in an area, you should take a moment to clean any track in the area just in case. Glue can be a great insulator, but its easy to clean.