Rinse the flowers and drain on paper towel. Pluck the yellow flowers (about 80 grams; approximately 3 ounces). Peel the lemon and squeeze. Place the petals, sugar, lemon juice, 125 ml (approximately 0.5 cup) of water and vodka in a tall jar and seal. Let soak for 4 weeks in a bright place. Filter the liquid through a coffee filter and into a carafe before serving.