I love this craft! It is awesome for boys because they enjoy anything gross. We used it when doing our Moses lesson, talking about boils being one of the 10 plagues in Egypt. The instructions can be found at Crafting the Word of God (which might just be one of my new favorite blogs.) If you want the kids to do the entire craft in one sitting, make the glue outlines on the back ahead of time. Mine took most of the day to dry. Also be sure to make a sample so the kids understand what is supposed to happen with the oil letting the boils show through.
Think of all the instances where boils are mentioned in the Bible–this could be used for any of them.