This is called a Swedish flame. Make your cuts like you're cutting cake, leave about 6 inches at the base, throw about a cap full of fuel oil in it. It burns up to two to three hours Can use olive oil instead of fuel oil :-) Any oil will burn!
Floating Flower Tip - Most flowers float pretty well on their own until they start to pool with water and slowly sink. For a longer lasting centerpiece, use bubble wrap to make little skirts for your flowers.