How do I write about someone being burned alive?

Im writing a story and a girl is burnt at the stake. If you don't know what that is, it's when people (usually a church) tie a witch or heretic to a large wooden pole, or "stake". There is usually wood around them, which is lit on fire, and the person is burnt to death. I Really need to write this with as much detail as possible. Please tell me how to do that.