Usually the GS gets covered in silicone after it's dry. Applying silicone to small areas then sprinkling coco bedding on the wet silicone to get it to stick there. Usually a small area is done at a time because silicone dries quickly. You could pretty much stick anything you wanted to the silicone at that point (small gravel, sand, etc...)
Do a keyword search and you'll get to see other people's pics they posted step by step, when they built thier vivs.
also this:
HabaCrete Vivarium Mortar Instructions.