Some people get a big lump of flint, wrap it in cloth to stop the shards and wallop it with a ruddy great hammer! I haven't found great big lumps yet though, so I tend to try and create a sharp edge by knapping, although I have had no formal training in knapping at all and heva almost zero skill in the process! I understand the basic theory though, so can get an edge that will cut and works well with a flint. As long as it is an acute angle, the finer the better, it should work fine.