Is it important to have a flat hone? The following pictures show lapping a barber hone if it is required to create a flat surface on which to hone a razor edge.
https://i.imgur.com/kqW6H1n.jpg
https://i.imgur.com/WRWIylr.jpg
Draw a pencil grid on the hone and start lapping. Keep lapping until all traces of the pencil grid are removed.
https://i.imgur.com/geSccBg.jpg
https://i.imgur.com/p3or568.jpg
https://i.imgur.com/GMswtHg.jpg
continued: