Start by holding the string/chain/whatever between your fingers. Usually it is done with the first and middle finger, but choose whatever is comfortable for you.

 

Now, there are many ways to do this, You may have to try different ways before you find one that works well.

Generally, we use a north south/ up down swing as a yes responce, and a side to side, west east swing as "no".

 

You can create a peice of paper that says yes / no, and let that guide the pendulum as it hangs from your fingers above the paper, or change yes, and no, to whatever you wish, choices perhaps.

 

Pendulums can also be used for finding lost items, you focus on the item, in much detail, until the pendulum swings, then follow it in an up down patter.

 

Again, the mind should be clear or focused before we start, a short meditation, or even just simply a few moments of gathering our "intent".