These are very simple to use and as such, are quite popular. You simply hold aloft the small end so the larger end can swing freely.

For divination, you must first ask the pendulum to show you which way it will swing for "yes" and which way for "no." Then, concentrate on your question. The pendulum should begin to swing, slowly at first, but then faster. Which way it swings indicates your answer. 

But, pendulums are easily influenced by the conscious mind, so for more accuracy, it is advised to suspend the pendulum, a "banana tree" (that snazzy little plastic or wooden device in your kitchen that you hang your bunch of bananas from) works well for this, that way you won't be touching it at all. Now, your subconscious , your Higher Self, is free to move the pendulum, without the conscious mind interfering, via the aura. 

Pendulums can also be useful for communication with spirits, finding haunted locals, healing, and realigning chakras.