Apply acid-free flux to both the outside of the male fitting and the inside of the female fitting. Flux further cleans the copper plus helps to prevent oxidation as the pipe heats up. If the pipe becomes oxidized, the joint may leak.
Join the two pieces securely together.
Unroll about four inches of solder and straighten it. You will use the roll or container as a handle when applying the solder.
Light the torch and apply the flame to the joint. Move the flame around to ensure that you heat the metal on the opposite side from you.
When the flux begins to bubble and spit, touch the tip of the solder to the joint. The solder should melt immediately and disappear into the joint. Remove the heat. Work quickly because the pipe's temperature will drop quickly. Move the tip of the solder around the entire joint to ensure that solder fills in all the way around. If the joint stopped taking up solder because the solder was not melting, then quickly add more heat so that more solder can be applied.
Once the joint will take no more solder it will build up outside of the joint and begin to drip. Use a damp rag to wipe the joint clean.
Always check for leaks after the pipe has cooled.
If you overheat the copper, it will oxidize and that prevents the solder from bonding.
If the joint leaks, you must open the joint, remove all the solder and start over by cleaning the metal and applying flux. It may be easier to start over with new fittings.
Always use lead-free solder.
Make sure the pipes are completely dry or it will interfere with the bonding of the solder.
If you cannot completely stop the flow of water from the pipe you are working on, it may be impossible to heat the pipe hot enough. Take a wad of white bread (without the crust) and stuff it into the pipe. This will hold the water back for a minute or two. After that the bread will dissolve harmlessly in the pipe and is easily flushed out.
If you use MAPP gas instead of propane, it burns much hotter and will heat the copper very quickly compared to propane. If you are used to propane, practice with MAPP before beginning work.