Insert a pipe expansion tool into an end of one of the pipes you want to join. Rotate the outer crank clockwise until the end of the pipe stretches wide enough to fit the mating pipe. This step is not necessary for air conditioner tubing that has expansions for joints.
Clean the insides and outsides of the adjoining pipe ends with sandpaper and a round wire brush. Rub sandpaper on the outsides and along the cut edges, and rub the insides with a wire brush.
Brush rosin flux over the outside of the smaller pipe end that will push into the expanded pipe. Brush flux onto the inside of the expanded pipe end.
Lay a fire blanket under the adjoining pipe sections. Put on work gloves. Push the smaller pipe end into the expanded pipe end. Light the open flame torch and heat the outside of the pipe joint for about 10 seconds.
Place lead-free solder against the top of the joint where the outer pipe meets the inner pipe. Apply pressure to the solder stick as you heat it with the torch until solder melts and drips all around the joint.
Wipe away the solder from the outside of the joined A/C lines with a rag quickly after you solder copper tubing. Clean away the excess before the solder dries and hardens.