This is so easy that I don't understand that I've been buying expensive commercially-produced seitan for years. Granted, it's a lot of work, and I still know one brand of seitan that's better than mine, from De Hobbit (but also very expensive).
A pound and a half of flour produces a large jam jar full, enough for a meal for five.
Make dough with the flour and enough water to make it easy to handle and knead until it's springy and supple (about 10-15 minutes). Let it stand for about an hour to let the gluten work.
Cover the dough with cold water in a large bowl (I use my kneading bowl) and knead it under water to flush out all the starch. You'll have to change the water several times. It looks as if you'll have nothing left at first, but eventually you'll end up with a spongy, doughy mass that squeaks a little when you squeeze it. Theoretically the water should run completely clear, but you're going to run into the law of diminishing returns if you try that. The flushing takes about half an hour.
Bring strong vegetable broth to the boil with a few slices of ginger, some cracked peppercorns (or ground black pepper) and a good splash of salty soy sauce. Drop the proto-seitan in as soon as it boils and reduce the heat at once. Ideally it shouldn't boil again, just simmer. If it does boil, the seitan gets spongy and breadlike (which is actually better for recipes with sauce to soak up, so you may want to encourage it).
After about an hour and a half of simmering, the seitan will have floated to the top and expanded a bit. Fish it out and bring the broth to a rolling boil to reduce it to about half. Meanwhile, cut the blob of seitan into centimetre-thick slices and cram it into the smallest glass jar it will fit in. Fill it to the brim with the reduced broth, put the lid on, and refrigerate when cool.
Seitan really needs to stand in the broth overnight to come into its own. It will keep for at least a week in the fridge.