While turning off the hot and cold water lines directly to the bathtub faucet is often sufficient, it's a good safety practice to turn off the main water supply as well. This prevents accidental water flow if the individual shut-off valves are old or fail, minimizing the risk of floods during the repair process.

The most reliable way is to remove your old valve stems and take them to the hardware store for an exact match. Pay close attention to the length, number of splines on the handle attachment, and the type of rubber washers or seals. Knowing your faucet's brand and model number can also help, but the old part is the best guide.

First, ensure all connections, especially the packing nuts and valve stems, are securely tightened, but not overtightened. Check that the rubber gaskets or O-rings on the new stems are properly seated and undamaged. Sometimes, a leak might originate from the faucet spout base rather than the handles, which could indicate a separate issue with the diverter or spout O-rings.

For a beginner, this project typically takes 1 to 2 hours, assuming you have the correct replacement parts readily available. The cost is usually low, primarily for the replacement valve stems, which can range from $15 to $50 per pair depending on the brand and complexity.