Zigzag hemmer foot 63V (3mm) is an expert when it comes to double-turned hems. The scroll on the front of the foot turns the fabric under twice, then sews it with a zigzag stitch.
Bernina 3mm Zigzag Hemmer Foot 63V - Sewing Machines Ireland
Sew a scant 1/4" seam allowance. Red lines mark 1/4" and 1/8" pivot points in front of and behind the needle. Straight stitch, length 2.0-2.5mm.