Clean the entire area that you will be drawing flagstones onto. You can use a power washer or a bucket of soapy water and a stiff brush.
Roll a very dark color of oil-based concrete stain onto the clean, dry area. Allow to dry according to the instructions on the stain.
Trace the shape of the flagstone mold onto the concrete with chalk. Repeat, moving the mold around until you have covered the entire area. Be sure to rotate the mold so that it creates a random look.
Apply a light-colored stain within the chalk lines to create the "flagstones." Allow to dry according to the directions.
Use a darker stain outside the lines to create the look of grout lines. Allow to dry according to the directions.
Use a medium shade to stain just the edges of the faux flagstones. This will create a realistic stone appearance, as flagstones can vary in color from the middle to the edges. Let this stain dry as well.