Thoroughly rinse the dandelion flowers under cold water to remove any dirt or debris. Gently shake off excess water and pat dry with paper towels.
Remove the yellow petals from the green bases of the flowers, discarding the green parts. Gently pinch the base of each flower and pull the petals away.
Place the petals in a large saucepan, cover with water, and bring to a boil over medium heat. Reduce heat to low and simmer for 30 minutes to extract flavors.
Strain the liquid through a fine mesh sieve or cheesecloth into another saucepan, pressing on the petals to extract all the liquid. Discard the petals.
Add granulated sugar and lemon juice to the strained liquid. Stir well and return to a boil over medium-high heat. Reduce heat and simmer until the syrup has thickened and reduced by half, about 1 hour.
Remove from heat, let cool slightly, then pour into sterilized jars, leaving space for expansion. Seal tightly.
Let the syrup cool completely in the refrigerator where it will thicken further.