Pendulums, like oracle or tarot cards, can be used as a form of divination; that is, helping to connect to your inner subconscious, Higher Self, or Spirit (whichever you prefer) in order to gain clarity, understanding, or perspective around a situation. Pendulums are unique in that you can use them to get "yes" or "no" answers, whereas cards generally give more general guidance; the flip side is that pendulums offer no further information or context, whereas cards generally do.
Although pendulums aren't something you should use to make every single decision in your life, they can be helpful in certain situations to gain better connection, clarification and awareness.
How to use your pendulum
Like any other spiritual practice, I believe it's best to listen to your intuition in terms of when to use your pendulum. There is no right or wrong in terms of frequency; use it as often or as little as you feel drawn to.
Sit in a calm, quiet space. It can also be a good idea to cleanse yourself, your pendulum, and your area first, using your preferred cleansing method. Take a few deep, cleansing breaths, then decide on how to read + connect with your pendulum. There are 3 ways it will swing: vertically, horizontally, and circular. Set your intentions, then hold the pendulum between your thumb and index finger, and ask "show me yes" and "show me no" or even "what does yes look like" and "what does no look like". Notice what each answer looks like - this is your key. You can also ask what "neutral" looks like, in the instance that there is no clear answer.
Start by asking questions you already know the answer to, eg "my name is XXXXX" and take notice of how the pendulum responds. This practice will help you connect with your pendulum; you can then move on to other questions.
As our paths are forever changing, it's not uncommon to receive one answer one day, then a different answer the next; nor do these answers reflect a concrete future. Remember that you are always in control, and you have the power to choose where to go.