The most overlooked part of tuning a chisel is flattening and polishing the back. The easiest way I’ve found to do this uses sandpaper and a piece of plate glass. Using a spray bottle, I spray water on the back of a sheet off sandpaper which usually creates enough suction to have it adhere to the glass. Depending on the condition of the chisel, I’ll choose the appropriate grit but generally I start with 80 grit open coat silicon carbide sandpaper. I move the chisel back and forth (along the long axis of the chisel), keeping the back, flat against the paper, until the marks on the back are consistent across the surface.