its a quite easy once you know what to look for after lapping in.you use two types of grinding paste, fine and coarse.
basicly a bit of coarse on the valve seat or valve and do the lapping motion with the sucker,and after a basic surface grind proceed to the fine paste.and what your looking for is a fine circular line in the valve seat and valve seal face,if not sure leave it to a engineering shop, you can remove all the paste with petrol afterwards
also when its done right you should be able to turn the head upsidedown pour petrol in the combustion chamber with the valves in position and none should leak through