Take cuttings of firm new growth apply 0.8% IBA (Rooting hormone)stick in well drained soil & mist. yakes 30-90 days to root

Common lilacs, or the French hybrids of them, are notoriously hard to root. Your best chance of success is with cuttings taken in early spring (first flush of new growth, before flowering) from a young plant. If you take cuttings from a mature shrub/tree, it is unlikely they will root.

If your lilac is a dwarf type (such as Dwarf Korean or Miss Kim), you may have better luck - but still from early spring cuttings. At the nursery where I work, we propagate lilacs in April.