In order to fire correctly, the gap between the two electrodes on the new spark plug must be properly set. Use a gap tool to adjust the side electrode (the one that hovers over the center electrode) until the gap is between 0.028" and 0.032".
Before installing the spark plug, cover the threads with a thin layer of anti-seize compound. This will ensure that the spark plugs come out easily at the next change.
Place the spark plug into the spark plug socket until it is firmly seated and reinstall it into the engine. Work slowly to avoid cross-threading. Tighten the plug until it is finger-tight and then provide an additional 1/2 turn with the wrench (about 20 ft-lbs).