We always recommend using a licensed plumber, especially when sweating fittings and valves.  That said, we are not to be held responsible if a DIY consumer follows our instructions and makes a mistake.  The basic steps would be to shut both water isolation valves which appear as red and blue screws on the side of the valve casting near where the water connects.  Use a flat head screwdriver for this.  This will isolate the water once the valve is installed and hooked up to water.  Remove the stem and threaded nipple, then remove the cartridge nut.  You should now be able to remove the cartridge by pulling it out.  You may need to rock the cartridge gently back and forth to loosen it.  Be sure all washers and o-rings are removed from inside the valve body before applying heat.

Once the valve is sweated and water is available, slowly open each of the valve stops with a screwdriver to flush out any brazing flux that may have gotten inside.  Once you are satisfied that all foreign material has been flushed out, you may reassemble the way you took it apart, then open the stop valves fully.  I hope this helps.