Measure the sugar into a glass jar with some extra room in the jar. Add the lilac blossoms to the sugar. Seal the lid tightly and give the jar a gentle shake all over. Store the jar in a cool dark place, shaking it once a day for at least one week. Remove any browned blossoms as needed.
Use a fine mesh strainer to sift the lilac flowers out of the sugar. Store the lilac sugar in the jar and use anywhere you would use regular sugar.
Notes
The moisture in the flowers will make the sugar clump during the week. Don't worry, just continue to shake every day to infuse the lilac flavor.