I've made quite a bit of DIY rock. It's fun to do, and you can make some cool shapes. I use damp sand as my mold in a large bin of some sort. I use the blue bag of rock salt at Lowes. But you can use any size you want. I honestly don't remember my ratios of salt to cement, but you would have to figure out what you want anyhow. I keep thinking 2 to 1 salt to cement. You probably want to mix in sand also, to strengthen the final product. Don't mix the salt until after you have already created your slurry of cement and sand. You can also throw in crushed shell if you wanted. You will have to allow your rock to leach out alkalinity for a month or so. If you have a creek or a stream nearby, that is the quickest way. Otherwise, put the rock in a bin of water in the backyard and change the water out every other day. When you don't see a film or scum on top of the surface, start to check your alkalinity levels.