My Aunt-- the woman who called me her Spirit Child, and taught me a little bit of many magickal paths as a child-- used a pendulum for much of her life. She even gave my cousins and I crystals that were appropriate for our age/size to use as such, and told us her method of figuring out the basic yes/no communication process.
A pendulum is a rock crystal, usually circular and with a pointed base, used to assist the user in determining the best answer to a given question. I've heard two basic belief systems for why this works: Either your guardian angel/spirit/goddess is guiding the crystal's movement; Or it resonates with your inner self, which has greater wisdom than your conscious self, and therefore has the ability to know the best answer for you in that moment-- and the rock moves accordingly.
I've also learned of at least three different ways to figure out the basic messages your crystal can give you. For my Aunt, there was a certain finger of your hand that you'd hold the pendulum over and this would mean "yes," and another for "no." I could never remember which finger was supposed to mean what-- and really, I think this was just a way for you to condition the pendulum to move in only one way for "yes." If you focused on "yes" and on the crystal moving, then it would make a "yes" movement. Another method is to write outcomes or chakras on a round piece of paper, divided like a pie. Whatever the pendulum sways toward is the answer to the question you've posed. No mater what, the most important aspect is to always use only one chosen method, and to remember that a pendulum only has one interpreter.
The way I did it was to ask a question to which I already knew the answer-- a "yes" answer. You have no idea how hard it is to come up with a truly yes/no question with no alternative interpretations!! I did the same with a "no" question-- and from there I knew what two movements to look for. I wore the stone in a pocket against my body for several days before doing this, to be sure I was in tune with my pendulum, by the way.
I've learned in practice that the faster my rock moves, and the more cleanly the movement is repeated, the more I can rely on that answer-- the more clearly an opinion has been expressed. I also don't have the crystals my Aunt gave me so long ago. I think I was six or seven at the time. And I found that a random semi-precious stone bead I'd picked up was the right thing for me to use anyway. It is not quite round, but has a definite point at the base. My stone is made of Amazonite.
Amazonite is a blue-green opaque stone, known to have a "powerful filtering action." It is said to open intuition, communication, and balance. It is used in healing and in filtering/connecting information and perspective for the best possible outcome. When I learned about these qualities of the pretty bead I'd randomly purchased... I understood why it would be such a great pendulum for me. And while I wear it nearly every day on a necklace, I don't use it to answer every single question that comes across my desk. I use my own intelligence and sense of rightness as much as possible.
I use my pendulum when there is no obvious way for me to know the best answer to a question before I make my decision. And then I have to be very careful about asking a question whose answer will be easy to interpret based on my skill at reading the movements. "Is this a good suppliment for me to take right now?" "Will this suppliment effectively heal this aspect of an illness I am experiencing?" (Two very different questions!) "Do I have access to something that would be better than this for that task?" (Now I just have to find it!) Asking if something will be good for you is a very general question-- maybe "good for you" is like those painful character-building activities of your childhood. Or maybe "good for you" would be a temporary kind of good-- as in a good person to learn the fine art of kissing from, but not a good person to marry...
Once I have a question in mind, I hold the item in my palm (or envision it's likeness in my palm if it is too big or not available), and concentrate on the desired outcome/the question as single-mindedly as possible. Sometimes my pendulum doesn't have an answer for me. Sometimes it takes a moment to decide. Sometimes it is not the answer I want to hear. But always, it is helpful to have this input from a greater knowledge than I consciously possess at this time.
If you've decided to look into pendulum work, I'd love to hear what kind of stone or crystal you've selected, and why. And if you know more than I do on the subject, I'd love to learn about a good book or other resource for learning more advanced pendulum readings.