You then ask a question. Usually the first question is 'is there anybody there'.
You will likely find that nothing will happen the first few times you ask the question. Sometimes you have to slightly nudge the glass just to shift it from it's 'seated' position but then stop pushing it immediately. The glass will appear to be moving under the power of someone, either otherworldly or not is up to you to decide.
Usually the movement of the glass to the word 'yes' is the first thing that happens. This usually then precipitates an accusing round of comments as everyone says 'you're pushing it!' and everyone saying 'no I'm not! Seriously!!!'.
Keep asking questions, and have everyone keep their fingers on the glass. You'll find that the more fingers on the glass, the stronger the reaction. It will get stronger and stronger as more questions are asked.
From time to time common occurrences will involve the glass deciding to spin round and around the board as if frustrated or angry. Sometimes the glass will claim to be a visitation from a relative or friend, other times it will claim to be 'EVIL'. Usually, I find the answers to be deliberately misleading.
Some people believe that it is a communication method with spirits, others believe that it is a subconscious movement of the glass by the people with their fingers on the glass. They don't realise they are doing it. Some people also believe that there is communication with spirits, but spirits that are there to ensure that there is no communication with the afterlife, and are intended to deceive and mislead any who try to do so.
If you're old enough, I usually recommend at least two drinks in you before doing it - it softens everyone up for the event. Turning off the lights and using candles around the room also adds to the atmosphere.
Finally, you are apparently supposed to put a lit match or hold a lighter under the glass at the end of the session to burn away or free any trapped essences. As I previously indicated - this is a matter of belief. Always leave the glass open-end up.