First shut off the main water supply. This is likely located close to your water meter where the main water supply enters your house. After shutting the water off, open a faucet on the lower level and let the water lines drain. Disconnect the water supply line running to the base of the fixture you will be working on. On some, this will just be a screw on connection, and on others it may be a soldered pipe. If it's soldered, use a hacksaw (or better yet, a pipe cutter) to cut the pipe in two places, about 2" down from the base of the fixture and also 6" to 10" further down the pipe. Use the reamer on the pipe cutter to remove any burrs on the copper pipes, and then clean them (with a piece of sandpaper or emery cloth) until they shine. Slide the compression nut from the shut off valve onto the pipe with the threads facing the open end. Then slide on the compression ring. Wrap the pipe with Teflon tape or spread plumbing joint compound on the end of the pipe, and then push the shutoff onto the end of the pipe. Slide the compression nut and ring up to the shut off valve and hand tighten the nut. Attach the compression fitting of the flexible supply tube to the other side of the shut off valve and hand tighten it as well. Attach the compression fitting on the supply tube to the base of the faucet and hand tighten it. Once all your compression fittings are attached and hand tightened, use an adjustable wrench to tighten them all firmly. Be careful you don't over tighten as this can bend the soft copper pipe out of round or damage a compression fitting. Turn the water back on and check for leaks. (Don't forget to open the shut off valve when you turn on the faucet.)
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First shut off the main water supply. This is likely located close to your water meter where the main water supply enters your house. After shutting the water off, open a faucet on the lower level and let the water lines drain. Disconnect the water supply line running to the base of the fixture you will be working on. On some, this will just be a screw on connection, and on others it may be a soldered pipe. If it's soldered, use a hacksaw (or better yet, a pipe cutter) to cut the pipe in two places, about 2" down from the base of the fixture and also 6" to 10" further down the pipe. Use the reamer on the pipe cutter to remove any burrs on the copper pipes, and then clean them (with a piece of sandpaper or emery cloth) until they shine. Slide the compression nut from the shut off valve onto the pipe with the threads facing the open end. Then slide on the compression ring. Wrap the pipe with Teflon tape or spread plumbing joint compound on the end of the pipe, and then push the shutoff onto the end of the pipe. Slide the compression nut and ring up to the shut off valve and hand tighten the nut. Attach the compression fitting of the flexible supply tube to the other side of the shut off valve and hand tighten it as well. Attach the compression fitting on the supply tube to the base of the faucet and hand tighten it. Once all your compression fittings are attached and hand tightened, use an adjustable wrench to tighten them all firmly. Be careful you don't over tighten as this can bend the soft copper pipe out of round or damage a compression fitting. Turn the water back on and check for leaks. (Don't forget to open the shut off valve when you turn on the faucet.)