Remove the high tension wires that connect your spark plug to the engine.
Use a deep socket usually the size of the spark plugs is (16mm) (18mm) (21mm) then unscrew the spark plug from the engine. Remove it one at a time for safety at use your high tension wire to get the spark plug completely.
Check the spark plug for any signs of dirt or corrosion. But if you see white or oil substances on the spark plug, it could be the sign of something more problem replace the spark plug if it happens again on the time you check the plugs there a bigger problem it may have a leak on the oil seal or the gasket.
Spray a cleaner or decreaser onto the spark plug.
Use wire brush. clean all around the spark plug's tip and also the tread of the plugs so that when you return the plugs it easy to tight at plugs.
Use the cleaner or decreaser again to the spark plug to blow away the dirty you've brush.
After cleaning it all put back all the high tension wires.