I have the same problem and have the above mentioned “simple fix” screws and tools. The problem is finding the joists which i have been unsuccessful at. I have a pretty good (new) stud finder with deep scan but its impossible to find the joists and this tool mentions that you must screw into the joists to secure the wood planks below the carpet, tightening up the subfloor and eliminating the squeak. Maybe im doing something wrong.