Wash the lemons. Cut away both the stem and blossom ends and slice each lemon into quarters. Cut away the thin strips of white pith from the interior of the wedges and use the tip of your knife to poke out the seeds. Cut each quarter into thin slices from top to bottom.
Once all the lemons are sliced, place them in a bowl (along with any of their liquid that you were able to capture) and add 4 cups of water. Let the sliced lemons soak overnight.
The next day, combine the lemons, soaking water, sugar, and ginger in a large, non-reactive pan.
Place the pan on the stove and set the burner to high.
Bring the contents of the pot to a boil and cook, stirring regularly, until the contents of the pot have reduced by at least half and are starting to shape up into marmalade. You may need to reduce the heat as cooking progresses so that you maintain a low boil without scorching the bottom of the pot.
While the marmalade cooks, prepare a boiling water bath canner and 5 half pints.
Test for set using both the plate and temperature tests (required temp is 220F).
When you deem that the marmalade is done, remove the pot from the heat.
Funnel the finished marmalade into the prepared jars, leaving 1/2 inch headspace.
Wipe the rims, apply the lids and rings, and process in a boiling water bath canner for 10 minutes.
When the time is up, remove the jars and set them on a folded kitchen towel to cool. When the jars have cooled enough that you can comfortably handle them, check the seals. Sealed jars can be stored at room temperature for up to a year. Any unsealed jars should be refrigerated and used promptly.