2 cups sugar
1/4 cup water
Place lemons into a medium/large sauce pan, cover with water (They float!), place over high heat and bring to a boil. When boiling occurs, reduce heat to a simmer, and simmer, stirring occasionally, for 30 minutes until lemons are tender.*
Drain the lemons, run under cool water, and allow to cool for 5 to 10 minutes to make them easier to handle.
On a work surface, cut the lemons in half. Remove and discard any seeds, then finely chop the flesh and peel. Place lemon into the saucepan along with sugar and water.
Cook over medium heat until the temperature on a candy thermometer reads 223°F, about 20 minutes (mine only took 17). Transfer to prepared jars and either sterilize the jars (if you plan to store them) or place into the fridge where it will keep for a month, if need be.
*As you can see from the picture above, one of my lemons collapsed a bit. I probably cooked them too long because I went the full 30 minutes. You may want to watch yours more closely than I did.
If you prefer a more complicated marmalade with a sophisticated taste, I recommend this Meyer Lemon Vanilla Bean Marmalade.