There are a few ways to trick a motion sensor, but the most common is using a piece of cardboard or other material to block the sensor's field of view. This will cause the sensor to think there is no movement, so it will not trigger the alarm.
Another way to trick a motion sensor is moving slowly through its field of view. This is because the sensor is designed to detect movement, so if something moves slowly through its field of view, it is less likely to trigger the alarm.