We have squeaky floors. Had squeaky floors. Last night before the contractor put up sheetrock on the ceilings today, S went downstairs while I jumped around in our kitchen and dining room and he de-squeaked our floors.
The whole process was super simple. I bounced around, S drew a circle from below where the squeak came from and then gently hammered a shim in between the subfloor and closest joist until the squeak stopped. Cut a 2x4 and screw through it and through the shim to hold it in place (but no so far you reach through the finished floor, I suppose.)