The petal is "painted" in on fine fabric with a single strand of thread. It's then cut out and applied to the main piece--Wow!
D's main craft is embroidery, particularly silk ribbon embroidery. Just now she's exploring stumpwork. This 3-D embroidery uses incredibly fine thread. Here's one panel of her sampler: That's a single strand of embroidery thread you can see draped across her fingernail. Here's the sort of thing she does with it: