When you begin the “gathering” process, it works best to pull the thread through from the bottom seam. BE GENTLE when pulling the fabric through the thread. Try your best not to break the seam.
It’ll save you a great deal of headache. If you do happen to break the seam, just sew another seam where the break was, making sure to pass the other seams with plenty of distance. Gather the ruffle to the desired size.