Rinse the dandelion flowers and drain well. Pluck the yellow petals (about 100 grams; approximately 3.5 ounces) and place in a pan with 500 ml (approximately 2 cups) of boiling water. Simmer for 6 minutes, remove from the heat and allow to cool.
Pour the contents of the pan through a fine sieve, pressing the petals well. Cook the liquid with the lemon juice and jam sugar for 4 minutes, stirring constantly. Pour the jelly into clean jars and seal immediately. Store until ready to serve.