All of the tubing and fittings need to be dry. Wire brush and sand all fittings and ends of tubing until they are shiny. Apply solder flux to all joints (both fitting and tubing). Make up the joint (put it together), and heat the fitting, not the tubing or the solder. Touch the solder to the seam where the tubing goes into the fitting. If it is heated properly, it will melt and the solder will suck down into the joint. Assume you know to use a gas torch, not a soldering iron.