Base center area of one section at a time, with a BIT of blending medium on your brush +Dioxazine Purple, Magenta & touches of Raw Sienna here & there. While still wet, use a small 1/4″ flat or filbert brush + White and starting in center of the section, make 4-petaled flowerettes. As you pick up the base color on your brush, move to the outer areas of the blossoms & continue to make flowerettes. You may overlap the flowers.
Here & there, pick up some of the DP and Magenta and tuck it in between the flowerettes.
When done with one section, move on to the next untill all sections are done and each lilac is one single blossom. Go back & repaint some of the highlight flowerettes with more white to enhance the rounded shape of the blossom.
Use the tip of one of your brush handles + Yellow Ochre to add centers of the flowerettes in the highlighted area.