Rinse lemons to get rid of impurities and pat dry.
Cut off lemon ends and discard, cut into two, and slice lemon.
Pick out seeds and discard seeds.
Place lemons into a large cooking pot with the sugar and mix up.
Bring to a rolling boil, reduce heat and simmer over a low to medium heat setting.
Reduce until lemons start to appear glossy and a bit translucent.
Blend with an immersion blender. I like it chunky.
Test if the marmalade is set by dropping some on an ice-cold plate. The setting temperature to check with a candy thermometer is 220 Fahrenheit/ 105 Celsius. If it runs, cook until set, repeat test.
Fill clean jars with your marmalade up to the rim. Close jar with lid and turn upside down to create vacuum. Turn it back down after 10 min or so.