As with a regular 'card' scraper, the cutting edge is a burr on the corner. Normally you form it by filing or grinding the edge square with the face of the scraper, and use a hard steel burnisher to compress the edge, which forms a burr, and turns it to more of a hook shape, which faces the right direction to cut properly. The hard scrapers are fully hardened steel, so you can't turn the edge with a burnisher. Instead you simply flatten the face and grind the edge to produce a burr.
Normally this is done by hollow grinding the edge after the face has been lapped flat on a fine stone. With a grinder you set up the tool rest so that the wheel hits the edge right in the middle when the tool is flat on the rest. The central plane of the tool is exactly on the radius of the wheel. Then you just run the scraper around, taking a light cut, until you've produced a burr all the way around.
You can also sharpen it by lapping the faces and standing the tool up on edge on the stone. This works best with something like a diamond stone, that maintains a pretty flat surface. A fine stone will produce a small, and sharp, burr that is good for fine cuts. I usually do this on my hard scrapers evey other sharpening, as it helps to keep the 'straight' edge straight.
A carbide skate sharpener should do the job of producing a hollow grind. It's a much smaller radius than most wheels, of course, so I have not actually had the patience to remove that much steel by hand.
The hollow grind is just a convenience anyway: it's fast, and makes it easier to keep the edge square. You could do the whole job on hand stones with some sort of guide to keep things square. A little out of square doesn't hurt, for that matter. The main thing is to lap the faces smooth and flat, and then grind the edge somehow. As with any edge tool, the smaller the scratches the sharper the edge, and the long it will last, all else equal.
Which brings to mind: I use I find it works best to hold the heavy scraper as nearly perpendicular to the face of the work as possible. This backs up the edge better. Hard steel is brittle, and it's microscopic chipping that dulls the edge. Leaning the scraper any further forward than necessary leaves less material behind the edge, and it dulls faster. I had one student who used the run off the end of the work with every stroke, and bang the tool into the bench top, dulling the tool in a few minutes, and making divots in the bench, too.
