Every engraver eventually comes up with their own particular way of sharpening Liners. Some of that depends on how one uses the Liner. Here are three photos that I use in my classes along with our discussions and hands-on use of the power hone and Dual Angle Fixture.
The first photo is a sketch showing that the face angle could be ground from 35 to 45 degrees depending on what the hardness of subject metal might be. The most important factor here is to see here is that the face is ground to a slight curve or radius. That allows the Liner to cut cleaner in curves and helps reduce the strain on the outer teeth.
The second photo is a close-up of one of my 18-10 liner better illustrating the curved face.
Finally, a photo of a selection of my Liners from narrow to wide showing in a side view of how the Liners are shaped.
Note: the Video listed in Sam's Post is also in my library - good stuff for anyone wanting to know everything about how to sharpen gravers.