Answer by
LeheckaG (1826)
Turn your hot & cold water-supply OFF.
Remove your old-bathtub-faucet,
probably remove handle with a screwdriver, faucet with a couple-of-wrenches
(one to firmly-hold your-faucet or water-supply-pipes, other to loosen (2)fitting's-hex-nuts).
Clean supply-pipe & new-faucet threaded-fittings with a wire-brush,
and apply either teflon-pipe-thread-compound or teflon-tape to your-fitting-threads.
Thread pipe-fitting-hex-nuts onto new-faucet, and apply bathroom-caulk between surface & new-faucet.
Turn water ON.