I don't understand the need for lapping rings?
You want the mating surface inside the ring to the scope tube to be a straight line, and not in an arc.
And as the photo Q.S.Y.B posted, you want front ring to be aligned with back ring. Quality rings won't make ring marks on a scope tube since it is two parrallel pieces of metal touching each other for the full length of the ring. "Lapping" is basically just like sanding wood, but you're lightly sanding away metal. And the lapping tool prevents you from getting out of alignment.
At least that's how I understand it all. Never have had to lap my own rings.