When I have a tough applique, usually I'll use a lightweight lining (lightweight muslin for example), sew right sides together, finger press seam, clip, and turn. I have an easier time clipping the edges properly that way. For an inner notch like the heart cleavage, I'll put a pin in where the bottom of the notch should be and aim for that when sewing. I used to always needle turn but I find this easier for complicated appliques.