we made garden stones for my man for Father's Day this year. we were inspired by a neighbour of mine who has started a community pebble mosaic project).
i was intimidated at first by the concrete, but in the end it was pretty easy.
the kids and i collected stones, beach glass, pinecones and leaves. we filled 3 disposable aluminum baking trays and half-way with sand. we wet the sand and then stuck our objects in and around the sand. (you are essentially working backwards- whatever is sticking down into the sand will be exposed on the finished stone, and whatever is sticking up out of the sand will be buried in concrete.)
i then carefully mixed and poured the concrete on top. we used plastic fridge magnet letters to write a message on the bottom of each stone before it dried.
my neighbour recommended wrapping each stone in plastic to cure for at least 3 days (i only did 3 days since we won't be walking on ours) then we turned the stones out and hosed them down to get the sand off.
my man was very pleased with these additions to our garden.