Begin by preparing the pipe to be soldered. The copper surface must be clean in order to form a good joint. Clean the end of the pipe on the outside with emery cloth or a specially made wire brush. Do the same with the inside of the fitting.
Apply flux -- a paste formulated to remove oxides and draw molten solder into the joint -- to the surfaces to be joined. Use a brush to coat the inside of the fitting and the outside of the pipe. Push the two pipes to be joined firmly into place so that they "bottom out" -- i.e., meet flush inside the fitting. To avoid over-soldering, bend the tip of the solder to the size of the pipe.
With the pipes and fitting in place, ignite the torch with the striker. (Some torches are equipped with auto-igniters, so a striker may not be necessary.) Wearing safety glasses and keeping the torch well away from your face, use the tip of the blue flame to heat the pipe ends and fitting.
After heating for a few moments, test whether the copper surface is hot enough by touching it with the end of the solder. If it melts, touch the solder to the joint seam, and watch for it to run around the joint. Turn off the torch and wipe any drips of excess solder from the joint with a rag.