Purchase several varieties of rock molds. They are typically made of latex or silicone, and often have open bottoms to pour a casting material into.
Set several molds in a row with their pour spouts facing up.
Spray the interior of each mold with a mold release spray to prevent the cement from bonding. Allow the mold to dry.
Mix a batch of cement in a mixing bucket, following the instructions on the bag. It should be thin enough to pour easily, but not so thin that it becomes watery.
Pour cement into each mold until it is full.
Tap the sides of each mold with your finger to free any trapped air bubbles, which will rise to the surface.
Allow the cement to cure for several hours.
Remove the cement rocks from the molds. The molds are flexible and will peel off of the cement.
Brush concrete stain onto each rock and wipe it off with a rag after a few seconds. Use several colors of stain to produce a realistic variety of rocks.
Let the stained rocks dry for 24 hours before handling them.