First I ordered a stencil from Bromley Craft Products together with a bag of Sandstone Brick Compound. Mind you, it is not quite as yellow as on their picture - but is slightly pinkish. I added some yellow acrylic paint to the mixture before spreading it on the foam board. I used a spatula and did not even out - just rough and as it comes. Like this I do not have an even stone surface.. some are a little thicker, others have 'creases', etc.
|If the rocks are not totally waterproof this is what will happen...|
Cover liberally (haha, as if there was a choice...) but scrape off excess.
Once the grouting is REALLY dry, use a sponge or micro fibre cloth to uncover the stones!