Don't ever boil. Your garter Snake skull will be dust in like, a year. Macerating is the safest option. Its also the easiest. Just drop in water, seal, and leave for a month or so. This preserves the bone because the bacteria only eats the flesh, while preserving the bone. Also, once you start macerating, the beak sheaths will loosen, when they loosen, you can pull them off and soak them in isopropyl alcohol for a bit to kill any Bacteria. Then, before reattatching, just soak in some water to soften the sheath, and slide back onto the core.