Clean the concrete surface area that you want to have covered with faux flagstone. You should use a water source with pressure, such as a garden hose with a high-pressure nozzle, a wire brush and some soap. This will help get any ground-in debris or stains off of the concrete so that the faux flagstone stain will adhere to the concrete surface.
Roll the darkest color of oil-based concrete stain on the entire surface of the concrete. This will be your base color, and will eventually serve as the faux grout between the flagstone. Allow the stain to dry thoroughly.
Place the flagstone mold facedown on the stained concrete. The wide, smooth surface of the mold should be against the concrete. You can obtain a flagstone mold at your local garden supply store or local nursery.
Trace the inside perimeter of the stone shapes with a piece of chalk. Depending on the size of the surface that you plan to make faux flagstone on, you may need to move the mold around periodically to cover the rest of the surface. Turning the mold in different directions will help you achieve a more natural stone appearance.
Remove the stone mold from the concrete surface and set it aside.
Paint the inside of the stone shapes with the lightest color of concrete stain using your paintbrush. You may need to apply more than one coat to completely cover the dark stain underneath. Allow to dry thoroughly.
Paint the edges of the stone shapes with the medium concrete stain color. This will create depth and help your faux flagstone to look more realistic. Allow to dry thoroughly.