Grind down parallel to the valve stem and perpendicular to the valve seats on the floor of the intake port where it transitions in to the valve seat.(the floor of the port is the "bottom" of the port when the cylinder head is normally installed - the port wall closest to the machined head mating surface. What I'm talking about is the area of the circular valve seat at the top of the picture. You do NOT want to undercut the valve seat. I'll post some diagrams sometime later.