Connecting two pieces of pipe is as simple as using a union to join the two together. However, if the pipes are two different metals such as iron/galvanised steel and copper, corrosion can occur with the slightest amount of static electricity that runs through metal. This corrosion will eventually lead to leaks. A special union called a dielectric union has two rubber gaskets in the centre that seal the pipes but prevents them from touching thus breaking the potential static electrical flow.
Unscrew the dielectric union to separate the two halves and set them on a table. The union will have a copper connector on one end (or half) and the other end (or half) will have a steel threaded connector. There will be two rubber gaskets inside the union where the two halves screw together to seal the connection.
Prepare the copper pipe to have the coupler soldered on. If needed measure and cut it to fit, using the hacksaw. Sand the end of the copper pipe to be joined to the steel pipe with the emery cloth. Sand about two inches of the outside until it is shiny all the way around.
Brush a thin coat of flux around the sanded end of the copper pipe. Slide one end of the copper coupler over the end of the pipe. Use the torch to heat the coupler and pipe connection until the flux begins to melt. Then melt the solder around the lip (or end) of the coupler where it touches the pipe. Allow the heat to pull the solder into the connection, sealing it.
Wrap one layer of Teflon pipe tape around the threads on the end of the steel pipe. Start at the end of the pipe and wrap the threads as you work your way up the end of the pipe.
Thread or screw the female steel connector from the dielectric union onto the taped threads on the pipe, Tighten with a pipe wrench.
Take the remaining half of the union that will connect to the copper pipe, and brush flux onto the end of the copper adaptor. Slide this end into the remaining end of the copper coupler from step 3. Use the torch and solder to seal the connection.
Insert one gasket into the steel half of the union and the other gasket into the copper half. Thread the two adaptors together and tighten them with the wrench.
Dielectric unions come in various sizes based on the outside diameter of the pipe, so it may be help to have a sample piece of pipe with you when you buy the union. Unions can be bought at a hardware, plumbing supply or building supply store.
Use caution when operating the torch, as the heat and tip can cause serious burns to the skin or fingers. After soldering, use caution as you wipe the pipe clean of flux as the pipe will retain heat for a few mintues. The heat could burn the skin.