Shut off the supply of water to the pipe that is experiencing the leak. You may be able to isolate the pipe with an individual shutoff. If you can not, shut off the main water supply for the house.
Drain the pipe by opening a faucet that is down-line from the leaky joint. Let the pipe drain until there is no water coming out.
Wipe the pipe dry of any condensation or moisture with a dry rag. Place goggles on your eyes to protect them from the soldering process.
Heat the joint with a propane torch. Hold the torch on the joint for two to three minutes to ensure it is hot. The hotter the copper is before applying the solder, the better the chance of getting a solid bead.
Touch the tip of the solder to the joint and let the solder roll around the circumference. Keep the torch on the joint but not directly on the solder. Move the tip of the solder around the joint to ensure the entire joint is sealed with a bead of solder. Use a wet rag to cool the joint. Turn on the water supply.