You don't have to spend a lot of money on expensive flagstone to get the look.
1. Get started by thoroughly cleaning the area to be stained. Use a power washer, a garden hose with a high-power sprayer, or a bucket of water with a little dishwashing soap or trisodium phosphate (TSP) and a stiff brush. Use a cleaner with a mildewcide if you live in a humid climate. Gather your supplies, including the mold, stains, and utensils. If sand-finish additive was mixed with stain, you may need to stir the stain often to prevent the sand from settling.
2. Brush or roll the darkest color of oil-base concrete stain onto the area. Let dry according to manufacturer's instructions. Place the mold, top side down, on the concrete. Trace the shapes of the "stones" with chalk. Move the mold around until you've covered the area, turning it for a random look.
3. Select the lightest color of stain and apply it inside the chalk outlines to create "stones." You may need to apply stain outside the lines to achieve larger "stones" and narrower "grout lines." Allow stain to dry.
4. Stain the edges of the "stones" with the medium shade. Experiment with the light and medium colors to achieve the right balance of shadow and highlight. Let stain dry. Make sure you allow extra time before walking on the surface if the weather is humid.