Cut off wisteria branches with flowers in spring, when the plant is in full bloom.
Hang the branches upside down with string in a dark, dry, cool area such as a closet. Let the branches hang until the flowers are completely dried out, which can take two weeks to a month.
Pick off the dried flowers, and crush them in a mortar and pestle. Fill a dark glass jar with the crushed flowers.
Cover the flowers with grape seed oil, a light, low-fragrance natural oil.
Cover the jar, and let it sit in a dark area for at least two weeks to allow the scent to develop.
Strain the mixture through a piece of cheesecloth into another dark glass jar to remove the solid flower pieces. Store the essential oil in the dark glass with a lid. Clear glass lets in light, which can deteriorate the oil.