If you want to learn how, i suggest you start with practicing on a psi wheel of how to psychically move things, but you do not have to to have a pendulum that works.
I found a good lesson online that can help you learn how to use a pendulum:
Pendulum work is a form of scrying. To interpret the deeper meaning of symbols and come to a conclusion is to scry. Descry (the word from which scry is derived) is an Old English word which means to see darkly or make out dimly. The Ouija Board, Black Mirrors, Pendulums, Candle Flame, Water, Crystal Ball and on and on...etc... are all different mediums to use for scrying. Scrying is when the person conducting the psychic work sees, hears, feels and "knows" information gathered in the mind and through the senses while focusing on a reading. The concept is that we are all connected to Universal Knowledge and that we can access that information. Some do it through deep meditation such as going to the Akashic Records in their minds and others use tools such as the pendulum to get more physical answers to their questions.
In the case of the pendulum it is a good medium to use to get quick answers to your questions as being connected to everything that is you already have the answer within you.
Simple instructions for using the pendulum:
1. Obtain a pendulum; this can be any pendant or object that dangles from the end of a string, chain or thread. Pendulums may be purchased or crafted by you. The important part is that it swings freely and easily and that you like looking at the object.
2. Get a good feel for your pendulum. Spend some time feeling the swing and movement of the pendulum. Find a comfortable position to hold your pendulum for extended periods of time. Some people rest their elbow on a surface with the forearm up and the wrist relaxed grasping the end of the chain between the first finger and thumb. Others hold the chain end of the pendulum to the third eye and allow it to connect directly to that psychic center while the pendant end dangles freely to answer your questions. Other ways to use a pendulum is to suspend it over a bowl of water (moon and feminine qualities) or sand and allow shapes, designs and symbols to form in the sand to enhance and expand your reading.
3. Determine Yes, No and Undetermined motions for your pendulum. After you have a "feel" for the movement of your pendulum it is time to ask the pendulum to show you the motion for a "yes" answer. This can be a clockwise swing or a straight line movement or counter-clockwise movement. It is all very personal. Do this for all three answers you may get: yes, no and undetermined. Once you know the directional movements for your answers you are ready to ask your questions.
4. To ask the questions some feel it necessary to cast a circle and sit in ritual. I personally prefer a more informal setting with a candle and quiet space to center and focus. Once you feel ready and calm begin your questions. If there are a lot of questions to be answered it might be good to have the questions already printed/written out on paper and have a writing instrument nearby to record the answers you get.
5. Also record any visions, ideas, thoughts or sensations you get while using your pendulum. This can be where the deeper, more substantial and meaningful answers come from. Sometimes when searching for a missing person or object the pendulum may be swung while visualizing the location. When you get a visual fix on a location you can ask the pendulum to confirm with a yes or no if you are on the right track.
The nice thing about a pendulum is that it can be small and be carried with you easily as well as used discretely. If you use a necklace from around your neck, to strangers it only looks like someone deep in thought twirling their necklace.
Pendulums can also be used to find an underground water source. Usually you would use one made of iron or steel for this. You hold it over the area you are searching and steady your hand. If there is a stream or large water source under the ground it will start to spin in a circle. This is a wonderful trick that my mums friend taught me when I was young :)