Gary, when you have access it's pretty easy to do. First, locate the squeak. Have someone walk or just put pressure on the floor from upstairs while you try to locate the exact area where the nails are moving in the wood and making the noise. When you have that narrowed down, the idea is to stop the wood from moving. It's the wood moving against a nail or nails that is the source of the noise. About any way you can install a block on a joist from underneath and run a screw into the bottom of the floor to pull the floor back down is acceptable, except that you do not want to use screws that are so long that they go completely thru the floor from below. Commercial units to do this easier are available.