Below are my personal preferences in materials to make terrain using the molds. However, for detailed casting instructions on how to use siicone molds, please follow the link below to the HIrst Arts instructions pages
Recommended pouring materials
The molds are very durable, however some materials can wear them out quicker. To get the longest use out of your molds, I recommend using some form of dental stone or plaster. My preferred medium is Merlins Magic due to its strength. Excalibur is also another really strong stone that works well.
Depending on what your preference is and what you are trying to achieve there are various options for materials to glue your molds to. Personally I prefer that my final projects can align close with Dwarven Forge terrain pieces. Therefore I use cork board which I purchase in 4 packs - 12" x 12" x 4.7 mm from Walmart. This just happens to be the least expensive place I've found that sells them. When gluing the standard 1/4" thick tiles to this cork board the tiles align really nice again the Dwarven Forge pieces.
As you will see on the Hirst Arts instructions, pink insulation foam is also a good alternative if you are okay with the added height.
My personal prefernce for gluing the dental stone tiles together is using Aleene's Original Tacky Glue. This too can be picked up at Walmart for a fairly reasonable price. Other PVA glues can also be quite effective, it just comes down to personal preference and availability.