Turn off the water supply
Drain the pipe completely
Cut the copper pipe cleanly
Deburr the cut ends
Clean the pipe and fitting with emery cloth or a wire brush
Apply flux to the cleaned pipe and fitting
Assemble the joint fully
Heat the fitting evenly with a torch
Touch solder to the joint, not the flame
Let the solder flow into the joint
Remove heat and allow the joint to cool
Wipe away excess flux and solder residue
Turn the water back on and check for leaks