I flatten the back sides of my corner chisel same as you would any chisel. Then I created a block of wood with the correct angle cut on the end as a file guide. Then clamped the block and chisel in a vice and held it in an appropriate position so that I could file it with a hand file, first one side then the other. It took a while but came out sharp and I'm happy with it. Jim
PS. I might have some corner chisels for sale soon, a blacksmith friend of mine has some, and he's suppose to bring them over on or before this Saturday. I'll let you know, when they are here.