To gap a spark plug, use a spark plug adjustment tool to gently bend the bottom electrode of the spark plug toward the other electrode to narrow the gap or away from it to widen the gap. Before adjusting the electrode, test the gap's width using a feeler gauge.
To test the width of the gap before adjusting, use the blade of the feeler gauge that matches the proper measurement of the gap. Pass this blade through the gap. If it does not touch any electrodes, the gap is too wide. If it cannot fit between the electrodes, the gap is too narrow. If the tool is able to pass through the gap with some contact to the electrodes, the gap probably does not have to be adjusted. Another option is to use a spark plug measuring tool to measure the gap of the spark plug to see how it compares.
When adjusting the gap, make sure not to put too much pressure on the electrode as it can break. If the electrode breaks, a new spark plug needs to be purchased. After adjusting, remeasure the gap to make sure the measurement is correct. If it is still not correct, continue using the spark plug adjustment tool until the proper measurement is attained.