There are a number of ways but you need access to either the top where you walk on or underneath.If you have no carpet on the stairs stand on the edge of each tread to find the squeak and drive a screw down through the tread into the riser(2 are better)in the old days when stairs wern't covered they would brush on french chalk or talcum powder along all the joints between tread and riser and into the end joints.If you can only access the underside of the stairs glue and tap in thin wooden wedges where the gaps are,it's basic and simple like driving a nail into a squeaky floorboard,dont be frightened of it(you could use ring nails instead of screws).Good luck.
Joiner, October 2010
