you need: lemons (a lot), sugar (almost as much), water, time, a heavy pot, a clean jar, and someone to call when it sets.

slice the lemons very thin. put them in the pot with the water. simmer until everything is soft and the kitchen smells like a greenhouse. add the sugar. stir. wait.

the trick is the wait. it will look like syrup for a long time. then, all at once, it will be marmalade. you'll know.