Yule ashes are a very powerful spell additive, boosting the power of any spell.  In order to have Yule ashes, you must clean your fireplace of all ashes before burning the Yule log.  Burn your Yule log, as your custom dictates, and when the ashes have completely cooled, collect them and place them into a container.  I live near Houston, Texas, USA, and it’s not often that it gets cold enough in December to burn a Yule log.  However, I had opportunity to do so in 2010, and still have the ashes left over from that.  I use mine sparingly, for obvious reasons, because I don’t know when I’ll have another opportunity to burn a Yule log.