I am sorry to be discouraging but I tried for many years to make a bonsai from a lilac. It was in a shallow 12 inch pot. It kept dieing back to ground level . The leaves eventually after ten years did become much smaller but it never did flower. finally after about 18 years it gave up. I am trying a different kind of lilac now that has smaller leaves and relatively tiny flower clusters but it is only 2 years old now. Maybe another member has had better luck with regular lilac. Mine was growing in the earth that I found it in in upstate new york. Maybe I should have worked in more drainage and fertilizer. 2nd guess. Ric