Make sure the valve and seat surfaces are clean, apply grinding paste evenly to the valve surface.
Using a suction cup, begin lapping with an oscillating motion, occasionally lifting the valve to prevent ridges from forming. Clean the valve and seat surfaces frequently and continue until a uniform grey stripe of approximately 1.6 mm wide forms on the valve and seat. Wipe off the grinding paste and consider the job complete.
Don't be too zealous when lapping the valves. If the valve and seat surfaces are not in satisfactory condition after fifteen minutes of operation, you will probably do more harm than good by continuing to lap.