Using a 6mm (3/8") seam allowance, stitch all around, leaving a 10cm (4") gap between the notches on one side only. Don't forget to backstitch.
( Tip: You may need to lift the presser foot to help you change direction when sewing some curves. Make sure you have the needle down in the fabric when you stop and lift the presser foot - that way you won't lose your place!)