I start by getting aquarium stones and a suitable base on which to build the master wall.
After wall has been constructed seal it with a layer of PVA glue. If there are any bigger holes in the wall close them with some wall filler.
I stick the masters onto a flat surface, a floor tile works well for this.
Next I construct a frame with Lego blocks and hot glue it to the tile. The inside of the frame containing the master is then sealed with a layer of rubber latex.
When dry the rubber latex is poured into the frame covering the masters. This is then set aside to dry. When all is dry remove the frame to expose the molds, remove the masters from mold and start casting.