Pull the blossoms from all the green parts and discard green parts. Wash and dry. I used my lettuce spin dryer and then laid them out on a pan for a couple of hours. Toss with sugar and put in a jar with a tight lid. I will let this sit for about a month.
UPDATE: I didn't have the patience to wait until the washed blossoms were completely dry so now I have very crystalline sugar. I am going to keep it (why not?) and whiz it in the blender. Now I have unwashed lilac blossoms pulled from the green parts in a jar of sugar. Only one bug so far!
Then let your imagination run wild. Floral scented sugar has so many interesting applications. Think about tea!