I'm following this NYT recipe, with a few methods modifications as explained here and with Jaime's ladle-into-a-pyrex trick. The resulting procedure is loads easier than most other marmalade recipes I've seen: nothing complicated with saving the seeds, nothing time-consuming with letting the fruit soak overnight, nothing dangerous or awkward with hot jars. Plus it's delicious! As well as absurdly pink.
These amounts are intended to be equivalent by volume to 1 part orange slices, 1 part lemon slices, 2 parts water, and 1.5 parts (or a touch more) sugar, to make 2-2.5 parts marmalade. If your fruit doesn't add up to 4 cups, just scale the other amounts accordingly.
Also, from experience, your marmalade-making experience will go much more smoothly if you use a large pot. Like a really large pot, one that can hold about 3 times the volume of what you're putting in it (so >12 parts worth). It's quite a pain in the ass to scrub nicely caramelized marmalade out of a gas burner after your pot boils over. (But this is the batch that went flawlessly, thankfully.)
2 blood oranges
7 Meyer lemons
4 c water
3-3.5 c sugar
spoons, saucers, canning jars with lids, pot, thermometer, ladle, pyrex or other jug that's good for pouring
Prepare the fruit by slicing off the ends, cutting the fruit in quarters lengthwise (parallel to the inner pithy core), picking out the inner pithy core and any visible seeds, thinly slicing each quarter crosswise (so you can see several segment bits in every slice), and picking out any remaining seeds. Discard the seeds, pith, and ends.
Put a few saucers and a few spoons in the freezer, put your jars and lids in a loaf pan or two so the jars stand upright, and preheat the oven to 225F. This will all make more sense later.
Place the fruit and water in a large pot and bring to a boil. Simmer for 20 minutes, until the peels are soft (erring on the chewy side of soft, rather than the disintegrating side). Add the sugar and bring back to a low boil. The oven should be preheated by now, so put the pans of jars in to sterilize. Every few minutes, give the marmalade a stir and check the temperature. When the temperature gets up to about 217F, after half an hour or so, start checking whether the marmalade is done by scooping out a spoonful with one of your chilled spoons and letting it sit in the freezer on one of your chilled saucers for a couple minutes. The marmalade is ready if the spoon of marmalade gels and gets a wrinkly skin on top; if it's still runny, keep boiling and stirring and waiting and checking.
When you decide the marmalade is done, turn off the heat and remove the pans of jars from the oven. For each jar, ladle a jar's worth of marmalade into your pouring jug, pour the marmalade into the jar, place the flat part of the lid on top of the jar, and loosely screw on the screwy part of the lid. Wait for that satisfying popping sound of jars well sealed (but don't stand there waiting, it takes anywhere from a few minutes to a few hours). Screw the lids on the rest of the way, then store unopened jars in the pantry and open jars in the fridge.