Scrying is a form of divination that works more directly with the intuitive aspects of the mind than most. It depends on the images formed in a mind deliberately cleared of conscious thought, whether through meditative techniques or simple daydreaming.
Traditional methods of scrying often use reflective surfaces as a focal point, though these are frequently darkened to restrict distracting reflections. The shadowy movements thus reflected allow shapes to form, which the mind is then free to interpret according to its intuitive or psychic understanding of the situation. Traditional media for scrying include vessels such as cups, bowls, or basins, often dark-bottomed, containing wine, water, or some other liquid, possibly with a film of oil on top. Or the scryer may choose a polished or matte stone or crystal, which may be left natural, or shaped or worked in a number of forms, including the well-known crystal ball. Even daydreaming images in the clouds, or a stucco ceiling can be a means of scrying. Alternatively, a person may scry in the flickering light of the fire, or a candle.
This last runs quite close to trance work, and is distinguishable based on where the images or information appear to come from; if the candle is lost to view, and the person appears to be looking at pictures "inside their head", or elsewhere in the world, I'd say this is a form of trance work known as clairvoyance, or "remote viewing". If the candle remains part of the scryer's awareness, and the images appear to arise from or in the flame, even if they gain detail far beyond what can be explained by simple patterns in the flame, I'd call it scrying. This distinction holds true regardless of the scrying medium. I mention it in connection with candle scrying simply because staring into a candle is one of the more common methods of inducing a trance state.
In either case, the images should be related to the subject most important to the person scrying, though whether it is because it is uppermost in their mind, the subject of their focus, or simply "what they need to know" is debateable.
For starters, it helps, for most people, to have the room dim. Full darkness interferes with the ability to see the focus of the scryer's gaze, which is the trigger for the images. Bright light tends to cause the immediate visual input to "drown out" the images the scryer seeks. What is sought is a light that will allow the eyes to unfocus without strain, and reduce distraction from the environment. Candlelight or firelight, or moonlight, are all good options.
The next step is to relax, and allow the gaze to rest gently on the focus being used, whether it is clouds, smoke, fire, mirror, stone, or liquid. As you hold the object in the center of your gaze, you can drop into scrying in one of two ways; either unfocus the eyes, holding the focus in the center of your visual field, but relaxing your eyes as if looking far beyond it, or focus on it until it fills the whole of your visual field, much as you might do when trying to thread a needle which has a very small eye. Whichever option you choose, remain relaxed, and breathe deeply throughout the scrying process, allowing the mind to empty of deliberate thought. Let the perception of the scrying focus fill your awareness.