A 62-year-old man with a 15-year history of primary varicose veins of the left lower limb presents with progressive symptoms despite conservative management. Duplex ultrasonography confirms saphenofemoral junction incompetence with reflux >0.5 seconds, and the greater saphenous vein (GSV) diameter is 8 mm at the groin. He is planned for saphenofemoral ligation and stripping. Intraoperatively, the surgeon identifies a small branch of the GSV arising 2 cm below the saphenofemoral junction that drains directly into the femoral vein. What is the most important step to prevent postoperative recurrence in this scenario?