Go to floral supply shops or places like Michael’s Craft Stores and look for artificial plants for flower arraignments. Some small leaf types of them can be used almost verbatim for trees and shrubs.
Look up how they did scenery way back in the 50’s and 60’s, making plants and trees using wire armatures colored sawdust and ground up cleaning sponge, all for dirt cheap. Korm’s methods are similar to this just scaled up.
For ballast, kitty litter, the NON CLUMPING type, thats VERY important, you want OLD SCHOOL Johnny Cat clay litter, and I use a container filled with water that has had a water based paint of the color of your choice mixed in, (10-20%paint to 80-90% water) I then used a old time cooking strainer to dip the litter into the water, all you have to do is get it wet, then lay it out to dry on a flat surface, preferably in warm sunlight and let it dry out. Objectively this is best done on a hot summer day. Laying it is as easy any other scale, get it down then use a 50-50% white glue/water mix to fix it down.