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Measure the length of your old tub faucet with a measuring tape and write it down on a piece of paper. This will help you purchase the same size and style of faucet that you had before. If you are installing a screw-on faucet, there should be threads on the end of your water supply pipe. Wrap 2-3 layers of Teflon tape around these threads to make it easier to screw on the tub faucet. Teflon tape is especially useful if the threads on your water supply pipe are stripped. Screw the new faucet onto the threads to attach it to the water supply pipe. Continue turning it clockwise until the faucet is tightly attached to the pipe. Line up the hole inside of the faucet with the water supply pipe and push the open end of the faucet onto the pipe. Turn the screw on the bottom of the faucet clockwise with an Allen wrench. Continue turning the screw until it's tight. This should secure your faucet in place.
Purchase a faucet that's the same length and style of your old one. Wrap Teflon tape around the water pipe if you're installing a screw-on faucet. Screw a screw-on faucet onto your water supply pipe. Fit the faucet over the pipe if you're using a slip-on faucet. Tighten the screw on the bottom of the slip-on faucet.