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.