How do you sweat brass pipe fitting?

First, clean the inside of the fitting to be soldered, and the outside of the tube it is to be soldered to, with abrasive cloth, steel brushes, steel wool or equivilent.

Second, apply a thin coating of soldering flux to these surfaces. Install the fitting on the piece of tubing and turn to be assured of a coating of flux is spread evenly between these surfaces.

Using a torch apply heat to the base of the cup of the fitting. Using a circular motion apply heat evenly all around the fitting (at its base). When flux begins to show signs of bubbling apply appropriate wire solder at top of fitting cup. If joint is sufficiently heated solder will become liquidious and run into the joint. Apply only enough heat to melt soldering compound being used. When solder becomes liquidious remove heat from joint. Allow joint to cool and solder to return to solidus state.

Overheating will cause flux to burn inside joint. The carbon formed will prevent solder from filling that area. If several of these "areas" join together, not only will the joint be weak but it will also leak!