Polish the outside of the pipe and fittings by using a wire-brush pipe cleaner. Polishing both surfaces is critical otherwise it would produce leaks later because the solder will not bond properly.
Apply plumber’s flux to the inside of each fitting to be soldered. Use flux brushes made for the task, as it keeps your fingers and hands clean. The flux has the consistency of a thin grease, and when heated will draw the solder into the fitting. Slide the pipe into the fitting, give it a twist to spread the flux evenly, and wipe off any excess flux that squeezes out. Now you are ready for the torch. Wear gloves when using a torch for safety.
To light the torch, open the valve slightly; you will hear a gentle hiss of the escaping gas. The nozzle is designed so that oxygen from the atmosphere of the room is drawn into intake holes, mixing with the propane. Light the flame using a sparking tool or open flame lighter. The hot point in a flame isn’t closest to the nozzle, as many people erroneously think, but rather about halfway along its length. Position the torch so that the flame heats the fitting directly (not the pipe). Hold the torch in one position, with the midpoint of its flame heating the section of the fitting that is farthest away from the joint to be sweated. That helps insure that the entire joint is hot when the solder is applied.
Let the flux be your guide: When it bubbles out and begins to steam, the melting temperature of solder has been reached. Touch the solder to the pipe. If it melts on contact, you can be sure it’s sufficiently hot. Remove the flame from the fitting before you apply the solder (it’s the heat of the fitting that melts the solder, not the flame of the torch). If possible, apply the solder from above so that the combination of gravity and capillary action can draw the solder into the joint. You may need to apply the solder to several points around the joint.
Do all the joints on the fitting at once (as on a T, with three pipes, or an elbow with two). The pipe remains quite hot for some minutes, so avoid touching it with your bare skin. After all the fittings have cooled, test the lines. If a fitting leaks, you can’t go back and just apply more solder, the joint must be reheated until the solder softens; then the joint must be pulled apart, the elements cleaned, fluxed, heated, and soldered all over again. I hope this helps.