Dump the concrete mix into a mixing trough. Add the proper amount of water for the specific mix that you are using. Mix the concrete in the mixing trough using a trowel.
Select a pigment that resembles the color of slate that you want to reproduce. Add the directed amount of pigment into the concrete mix for the amount of concrete you are mixing. Mix the pigment into the concrete with a trowel.
Fill the natural stone concrete form with concrete using a trowel. Select a concrete form that has the rough texture of slate. Allow the concrete to set up for the directed amount of time.
Remove the concrete from the mold. Allow the concrete to cure completely.
Seal the concrete with a concrete sealer to protect the slate look for your project.