Peel the zest from the lemons with a vegetable peeler, avoiding as much of the white pith as possible. Slice the zest strips crosswise very thinly at an angle to make strips about 1/16 inch wide by 1 inch long—you’ll need 1 cup of zest strips. Put the zest in a 4-quart (or larger) saucepan.
Trim the ends from the zested lemons to expose the flesh. With one cut side down on the cutting board, trim the pith off the lemon all the way around and discard the pith. Quarter the lemons lengthwise and remove any visible membranes and seeds. Slice the wedges crosswise 1/4 inch thick—you’ll need about 1-1/2 cups.
Add the sliced lemons, ginger, and 2 cups water to the lemon zest. Bring to a boil over medium-high heat, adjust the heat to maintain a simmer, and cook until the zest is soft and the membranes start to break down, 6 to 8 minutes.
Whisk the pectin into the mixture. Increase the heat to high, add the sugar, and bring to a boil, whisking constantly to smooth lumps. Boil vigorously for 1 minute, whisking constantly (move the pan off the burner momentarily if it threatens to boil over). Remove the pan from the heat and let sit undisturbed for 5 minutes.
Skim any foam and seeds off the surface of the marmalade. Stir gently to redistribute the solids. Transfer the marmalade to heatproof storage containers, let cool to room temperature, and then refrigerate for up to 1 month.
For longer storage at room temperature, can the marmalade. See the canning directions below.
To can the marmalade:
Transfer the hot marmalade to clean, hot canning jars, leaving 1/2 inch of headspace in each jar, and wipe the edges clean with a paper towel. Screw the lids on tightly.
Put the jars in a large pot of water fitted with a rack insert. The water should completely cover the jars by at least 2 inches. Return the jars to the pot of water and make sure the water covers them by at least 2 inches. Boil, covered, for 10 minutes. Use tongs to remove the jars; let them cool undisturbed on the counter. You should hear a popping sound as the jars cool, indicating that the vacuum seals have worked.