Steps
Clean out everything which is placed on, under, and around the bathroom sink. Spread a towel on the bottom of the vanity (if you have one). This will ensure that leakages and drips do not spread into the entire vanity. Turn off the water supply to the sink(s). Ensure that the water has completely stopped before you remove supply lines. Then, use pliers and remove the bottom faucet.
Once you have loosened the faucet, remove the water line which is coming from the wall, and clean all the threads with a towel, patting them dry in the end. Once they've dried, wrap all the sets with a pipe tape. Ensure that all the threads are covered with the tape completely. Now, thread the new supply line to the faucet, ensuring that you don't cross thread. Now, repeat this procedure on the water lines.
Once done, place a small bucket under the drain tap. This will ensure that water does not spill out. Start loosening the upper nut first. Then loosen the nut on the pipe, remove the pipe from the wall, and place it in the bucket. Remove all the old washers and nuts from the pipes. Do not throw any parts; if they are in good condition, you can use them again. In case broken, you can discard them.
http://www.youtube.com/watch?v=4AHD3m23808
Now clean the pipes carefully and then dry them well. Place the large nut with thread facing out onto the pipe (the one coming out from the wall). Place the washer with the beveled edge on the same pipe. Then place and hold the large nut on the drain, face down away from the sink, hold carefully, and place the washer with beveled edge down.
Then place the pipe dope around the threads of the new trap carefully. Slide the pipe over the wall and then from the sink. Then slide the large nuts to the threads and tighten them with your hands, until you are not able to tighten them any more. Using a channel lock or pipe wrench, give each nut a half turn, but not more than that, else they might break.
When you are going to start the project, ensure you have all the material you need. Before starting off with the project, it is important that you switch the supply lines off. It is a good idea to check the supply lines by leaving the faucet running, once you have shut the valves off. If you are using plastic nuts, you may not want to over tighten them, as there is a possibility that the nuts may crack.
http://www.buzzle.com/articles/bathroom-sink-plumbing.html
