Welcome to ATP!
The petiole is the stem that attaches the leaf to the plant. The stem is the part of the plant that all the leaves, with each leaf petiole, is attached.
Does that help? As I can't see where the leaf/petiole is attached to the stem, I can't tell you where to cut. If you are still confused, try to take a photo of the inside of the plant showing the main stem and leaf attachments.
Daisy