Mum used strong white flower, which made it easier to knead, but made the pastry rather leathery. Made with cake flour might be nicer, but apparently rye is rather hard to knead, so possibly worth doing in a machine.
Instead of potato, you could use a milk rice porridge (which I remember as being nicer, but my mum prefers potato and it's easier to do).
The dough is cut into little balls and then rolled out into circles. Fill centre with a thin layer of the filling, then fold in the pastry from one end, making a number of little decorative crimped pleats (not as difficult as we'd thought to make the traditional mocassin shape, but even easier to make round ones).
Baked in a very high oven (one recipe suggests 300°, basically as hot as the oven will go) for about 12-15 minutes, the filling will be slightly browned in parts). When it comes out, the pastry is ridiculously hard. You soften it in melted butter (we used butter melted in boiling water, some people also add milk), brush or dip them, then pile them on top of each other so that they soften.