Clean all valves from carbon deposits.
Make sure the valve and seat surfaces are clean, evenly apply grinding paste to the valve surface.
Using a suction cup, begin lapping with an oscillating motion, lifting the valve from time to time to prevent ridges from forming. Frequently clean the valve and seat surfaces and continue working until a uniform gray streak about 1.6 mm wide is formed on the valve and seat. Wipe off the sanding paste and consider the job done.
Do not be too zealous when lapping the valves. If the seat and valve surfaces are not in satisfactory condition after fifteen minutes of work, continuing to lap will probably do more harm than good.