cut off the stem end of the fruit, then split each lengthwise. slice each half into 1/8-inch moons, removing and discarding the seeds as you slice. place the kumquats into a small saucepan and just cover with water. add sugar and bring to a boil over high heat for 15 minutes, skimming off any foam that comes to the top. reduce heat and continue simmering until the marmalade thickens to the consistency you like. add the lemon juice and stir. let cool, then put in a pretty jar and refrigerate.