Thanks everybody for taking time to answer my question. Here is how I did it: First I used torch to heat the copper pipe and tried to remove residue of remaining solder with a clean sock(as damp cloth), then I used emery cloth and the other tool(cleaning tool with wire in it) to clean the pipe joint. I can tell that the copper pipe is shiny after cleaning. CY: Clean is essential.
I am not sure whether I need to clean the fitting of newly bought brass valve or not, should I? CY: yes. They sell brushes for doing this. About three bucks for the brush. Worth every penny. The inside surface of the fitting (valve) absoloutely totally has to be cleaned.
Next I applied the flux only to the copper pipe source, then placed the brass valve. CY: Then let it sit for aobut sixty seconds to let the flux work.
I don't have MAPP gas, only the propane gas. I heated the fitting until I can hear the sizzling sound, then I applied the solder. If I use propane torch, how can I tell that time is right to apply solder or in other word, when the fitting is hot enough to apply solder using propane torch? CY: Put the heat on the near side of the fitting. Curve the solder around, and touch it to the far side of the fitting where it meets the pipe. At the exact moment hwen the solder starts to flow, feed in at least an inch of solder. Any excess should drip out.
Can propane torch generate enough heat for solering brass valve? CY: Should be OK.
Thx again.