Examine the spaces between the flagstone pieces to see if they are filled with sand or gravel. If they are filled with gravel, use a trowel to scrape out all the gravel, which will prevent grass roots from growing.
Place a ruler next to one of the flagstones and make sure there is at least 2 inches of depth. If needed, dig out additional soil between the flagstones until there is a uniform 2-inch depth.
Lay a piece of grass sod in the gap between two flagstone pieces and run a utility knife along the edge of one of them to trim the sod to size.
Repeat the process to cut pieces of sod for each remaining gap between the flagstone pieces.
Sweep the tops of the flagstones gently with a broom to remove any gravel, dirt or sod. Be careful not to disturb the sod as you do this.
Walk on top of all the sod pieces to push them firmly into the ground.
Water the pieces of sod well with a garden hose until they are saturated. After installation, water the sod once or twice a day to prevent it from drying out until you see new grass blades develop.