To start, take a standard GW base and your 2 part epoxy, whether it be Milliput or the traditional Green Stuff and cut enough from each part so that you'll be able to cover the base. Mix the two parts together by kneading them together. Next spread your epoxy evenly across the base's top surface area.
Using a hooked tool, choose a point to begin your first crater. Pull the epoxy outward from that point. Use the back of a paintbrush to make a couple (2 or 3) deep indents. Move on to a ball point pen and make indents all around the the rest of the top area. Ball up small pieces to fit into indents made by the paintbrush. Wait 24 hours for the epoxy to cure.
Above are some Insta-Mold pieces. Using a cup of hot water (180-210 degrees), melt the pieces together. Kneed them together and cover the base.
Either wait about 5 minutes or put the base in a cup of ice water. After it has cooled, remove the mold and set aside the base you made. Now you'll be able to easily make multiples of that base.
Now comes the tricky bit. Resin epoxy heats up when it cures, so depending on how deep the Insta-mold casing is, you may need to mix some, let it cure, then finish it up with another mixing of resin. From my own experience this won't be so big to cause the resin to heat enough to melt the Insta-mold.
Mix your resin in a cup and pour it into the mold. Allow roughly 30 minutes for the resin to properly cure and remove it from the mold. Prime it and begin painting as you see fit! Below are the steps I took to finish my own base.
And there we go. A suitably alien looking terrain fit for any Daemon army.