Preheat the oven to 190C, 375F, with the usual adjustment for fan ovens.
1lb flour, brown or white, but I doubt self-raising is a good idea.
Mix with water (about a quarter pint) to form a stiff dough. Roll out to about 8mm thick and cut up into pieces - I used a pizza wheel. Authentic military hard tack came in 3" squares, but I cut it much smaller, since I expect this batch to be mostly used for tasting rather than as staple food. Poke dents into both side of each piece with a stick (I used the butt end of a chopstick). Place the pieces on an ungreased flat baking sheet. Bake for 30-40 minutes (it doesn't seem necessary to be precise), turn the pieces over and bake again for the same time, after which it should be golden brown. Allow to cool before sampling.
It really is very hard. "Toothbreakers" was one nickname, and it's only minor hyperbole. Once you break up a piece, you'll discover it's brown all the way through. It tastes OK, if unexciting: basically like extremely crunchy bread. I can see why military cooks would frequently grind it back to flour and make something more interesting.
It should keep more or less indefinitely in a cool dry place: I'm using an airtight jar, and hoping that weevils don't actually appear spontaneously.