These can be tough. They like to wobble and split a lot because they are so thick and thin in places. SIze 15 is huge for this coin. I usually do not go past 14 for a half dollar and this particular coin would want to split.
Annealing is taking the coin to a dull red heat in a pitch black room, letting it cool to a dark heat then quenching in water. . If you have any light on it, you will not see the true color. This is done as needed throughout the ring making process. The coin will work harden as you make the =ring, stretching and reducing the metal. I would suspect about 4 annealings would be sufficient for that ring size.
You will need to overstretch the ring 2-3 times past you target size to get a symmetrical looking ring, that means stretched to a size 17 minimum. You will lose some details because of this and most likely split the ring. Be sure to remove any and all stress risers (micro tears) along the cut edge to help keep it from tearing. Stretch in small increments, twisting the ring to place it on a different part of the stretcher each time and check for stress risers often.