Here's what I do.
If you haven't got an actual base you'll need to make that first......I use Urethane Foam - 2 Part Liquid 3# density foam to pour for a base. Mix equal parts and pour it over a core such 3/4" plywood. Use disposable gloves and Work quickly as it starts to set within minutes. Once it sets and hardens usually within 10 minutes you can cut and shape it as desired.
Once that is complete, rough up any shiny spots on the surface so glue can stick.
Now for the dirt, I use regular peat moss.
If you already have a base......First you will want to cover your urethane foam base with Elmers glue and lots of it. Pack the peat moss over the Elmers glue covered base. Once packed let dry, usually takes a whole day. After you think its dry, shake off the excess peat moss. If you find that you have missed a spot or two you can cover the area in Elmers glue and repeat the process. Don't worry the Elmers dries clear. After everything is to your satisfaction, mix Elmers glue and water in a spray bottle, (30glue/70water) mix it well and spray the surface of your base. This seals the peat moss to your base.