Locating and shutting off the main water supply is the most critical first step. If you can't find it, check your water heater for a shut-off valve, or contact your building manager or a professional plumber. Do not proceed with the project until the water supply is completely off to avoid significant water damage.

Yes, it's highly recommended to have plumber's tape (Teflon tape) for sealing threads, penetrating oil for loosening stuck or corroded old parts, and a small wire brush for cleaning debris. You might also find a basin wrench helpful for reaching nuts in tight spaces behind the tub wall.

First, re-check that the new valve stems are fully seated and securely tightened, but not overtightened. Ensure all O-rings are correctly positioned and intact, as a misplaced or damaged O-ring is a common cause of leaks. Also, verify that the connections to the water supply lines are tight and sealed with plumber's tape.

While 4 hours is a good estimate for someone with some DIY experience and no major issues, a complete beginner should budget more, perhaps 6-8 hours. Encountering corroded or stuck parts, or making unexpected trips to the hardware store for forgotten items, can significantly extend the project time.