Pour the sand onto the area where you plan to set the flagstone.
Spread out the sand with the back of a garden rake. Make it level so you will not have to spend as much time leveling when you are tamping.
Tamp the sand down with a tamper --- a flat piece of metal attached to a handle. Lift the tamper, then force it down onto the sand to compact it. Use a level to ensure you have leveled the sand.
Place the flagstone into the sand. Wiggle it around to set it firmly. Check whether the stone is level; if not, tap it with a rubber mallet in the direction that will make it level.