First grind squate tip when viewed from above.
Next grind slope of 25 deg
Sharpen on oil stone back and forth to make it flat
Now change angle to 30 deg and put on an edge about 2mm wide.
Change over to fine stone and continue 30 deg sharpening
At end you will feel a burr on the underside - where the thin metal has folded back on itself. Place blade flat on stone and move sideways a few times to remove burr.
Check out Trend's FASTTRACK sharpening jig. The chisel remains stationary and flat as you slide the sharpening stone back and forth across the face of the chisel. It has two specific grinding angles - 25 to start and 30 to finish and there are at least three sharpening stones available - coarse - fine & very fine. About £45 with two stones.
If £45 is too much at least invest in a wheeled sharpening jig to hold the chisel at a constant angle and try not to plough a furrow in the oil stone by following the same line all the time. Most of the "work" will be done on the push stroke but it matters not if you apply pressure on the return other than the oil stone will move towards you!
A lot of confusion exists about grinding angles - as far as I can make out 15 degrees is excellent but the edge has no strength - Craftsmen go for 25 deg but when it needs re-sharpening the whole face needs to be ground away - the easier solution is to grind first at 25 deg and finish with a thin strip of 30 deg. This way, when it's time to re-sharpen, you only have to grind off a small amount of metal to restore the sharp edge.
Richard Buxton
September 2008