An invisible zipper requires a 1 cm seam allowance which I've drawn in below for this demonstration. (apairandasparediy.com)
Cut around it, leaving a 1 cm seam allowance on the sides and 4 cm along the bottom. (idealhome.co.uk)
Using 0.5 cm seam allowance, starting from the point of the heart, sew around the heart, leaving a gap of about 2 cm in one of the long sides — stay stitch (stitching forward and back a couple of stitches) at the start, end, and either side of the gap to secure the stitches (boho-weddings.com)