Wash your quinces, rubbing off the fuzz. Then hack them into rough cubes, skin and pits and all — trim away any nasty brown or wormy bits, but beyond that it's all fair game. Toss the cubes in a large pot with enough water to cover by barely an inch or so. Bring to a boil, then reduce heat until it's just high enough to maintain a vigorous simmer. Cook until the quince bits are totally mushy-soft, and beginning to take on a rosy hue, about an hour (you can also do this in a pressure cooker).
Pour the mixture through a strainer to capture the quince cooking liquid (you can run the quince pieces themselves through a food mill, or simply compost them — they'll have given up most of their flavor to the liquid). Measure the resulting liquid, and for every quart, add the juice of a small lemon.
Pass the liquid through a single layer of cheesecloth or loose-weave dishcloth, then through a couple layers of cheesecloth (or a coffee filter), to end up with a clear pink liquid. This final straining can take a while. If you are lazy or late-starting like me, you can leave it to drip overnight, and make your jelly the next day.
When you're ready to make the jelly, pour the quince liquid into a wide-bottom pot, and add a generous 3/4 cup sugar for every cup of juice. Bring to a boil, then reduce the heat slightly to a vigorous-but-not-crazy boil. You'll need to cook the mixture down to the gel point, which should take at least a half hour and up to an hour, over which time it will thicken and darken to a ruby color, and the quince flavor will concentrate. When you think it's becoming syrupy, place a few small plates in the freezer to chill. If you want to process the jelly, prepare your jars and start a boiling water bath.
To test the jelly, drip a few drops onto a freezer-chilled plate. Let sit a minute, then push into the edge of the drop with your finger. You don't need to see a full jelly-like set, but it should just begin to wrinkle and mound ahead of your finger as you push it. When it reaches this stage, you're done! Pour into your prepared jars, seal, and process in a boiling water bath if desired. The jelly may take some time to set, but will end up fairly firm.