Before you can make a center, you have to have your flower. See how here. Don't have a petal perfect ruler yet? link here.
Why you should try adding centers to your flowers.
The framework is a single Petal Perfect flower using petal D. Beginning at the center mark, stitch into the petal creating a leaf design and return to the center. Stitch back into the leaf, creating a single vein, returning again to the center point. Five of these and your center is complete.
Link to other ideas