The Ouija Board can be a very useful tool in locating the ghost. This is not equipment you can buy and bring with you, however. It is something that you may find as you explore the building. It does not appear in every assignment and its location, when it does appear, is randomized.
To use the Ouija Board you'll want to turn off any lights in the room. Your flashlight and shoulder light don't matter, so you can leave those on. Simply turn on the Ouija Board by pressing E (default) when you are close enough. If it is on the floor you may have to crouch to reach it. When it is turned on it will light up. Two questions I have seen it answer are 'Where are you?' and 'How old are you?' The only answer of use is the location of the ghost.
It is possible that the answer will simply be that the ghost will turn off the board and possibly play with the lights in the room. If you get that answer you should keep in mind that your sanity has probably taken a hit and the ghost is most likely becoming angry. You can push your luck by turning it back on and asking it again. Sometimes asking it 'How old are you?' can get the ghost to be responsive and give you an answer to 'Where are you?' if you ask again.
Normally using a Ouija Board in Phasmophobia will drain your sanity unless the ghost type is Demon. Keep in mind that this is not a sure thing. I have seen sanity go unaffected by other ghost types and I have also seen a Demon cause a massive sanity drain when using the Ouija board. While this happens very rarely, a Demon can cause a major hit to your sanity if it refuses to answer the question.
Once you are done with the Ouija Board, you should turn it off. Every time it answers a question there is a chance that it will seriously drain sanity and could also become extremely aggravated. It can also interfere with getting responses from the Spirit Box and getting responses from other Equipment.
The Ouija Board in Phasmophobia also presents two Photo opportunities. The first is when you first turn it on before asking a question. This should give you a 'Ouija Board' photo in your journal. Get real close to take the photos or they may not count. The second opportunity is while it is answering your question. A photo taken up close while it is answering will usually count as an interaction.