In Royal Icing, on Flower Nail with a waxed paper square, pipe four tiny rounded petals with tip 59 or 101s. You can pipe several 4-petal groups at the same time if you like. Let dry thoroughly.
Make a base for the flowers in buttercream and arrange on your cake. Pipe a tip 32 upside down elongated shell. (The tail/tip of the shell will be the top of the lilac cluster.) Overpipe for more dimension if needed.
Attach petal groups to shells with dots of icing, covering completely, forming a lilac cluster of flowers.