You'll also need a few kilograms of some type of conventional explosive. C4 is excellent, but regular old TNT will work just about as well. The basic principle is to surround the plutonium with the explosive, and have this enclosed in a metallic sphere. (usually made of beryllium) It must be set up so that all of the explosive detonates simultaneously. This causes the force of the detonation to be directed inward, bringing the plutonium to critical mass and triggering a chain reaction known as fission.