So you want to learn how to use a pendulum! That is awesome because you can get many answers, from your soul level, when you decide to jump in to the pendulum life. ;)
First though... you may be wondering exactly how a pendulum works. This is up for debate between different types of people. Of course, the woo-woo side says it's your soul, or your spirit guides who make the pendulum swing. Then, there are the scientists who say it's based off of microscopic movements in your hand, and that you are actually the one making it swing. Of course, there are the sceptics, who say it's all hooey and that you're full of it and your the one making it swing on purpose.
Personally, I believe your soul is connected to your spirit guides (and the entire spirit world), and your soul is connected to your subconscious as well. So, in my opinion, it very well could be small, microscopic movements from your subconscious mind - but, of course, that would mean those small, microscopic movements are initiated by your soul/spirit guides/spirit world.
Whatever or however it works, it's pretty cool. The more you practice with a pendulum, the more your own opinion will develop as well. All I know is that I'm not full of it, and I am not making it move to fool myself and others.
In the directions below, you will see that you can use a necklace, a string with a heavy washer, or whatever you have on hand and use it as a pendulum. You want to bottom part to have some weight, but not be SO heavy it's hard to swing.
I prefer a necklace I have, over an actual pendulum; but there are some beautiful pendulums out there that a lot of people prefer. If you're in the market, I suggest checking out Etsy for pendulums. The people on there put their love and energy into creating wonderful pendulums.
It's also recommended to wear your pendulum (if you can), or keep it in your pocket. That way, it will become more in-tune with you and your energies.
How to Use A Pendulum
Find something that will swing, like a necklace or even a heavy washer tied to a string. (The more special it feels to you, the better.)
Quiet your mind, think of your question, and hold the end of your pendulum against your third eye (between your eyebrows and ever so slightly up toward hairline.)
Rest your elbows on the table.
Breathe calmly while thinking about your question.
Wait for your pendulum to move to reveal your answer.
The more you practice, the more you will build a relationship with your pendulum and build the trust within your self; so practice often!