Scrying is a form of divination, divination being a form of fortune telling, that utilizes gazing to receive messages or visions. There are many different mediums that can be used for scrying including water, fire, smoke, mirrors, crystal, etc.
In general, scrying is done with little to no light. What little light there is should either be from candles or the Moon. Although you can scry during the day, it is more often done at night. The key to scrying is to soft focus. Have you ever stared off into the distance to the point where everything in front of you begins to seem soft and unreal? That's what soft focusing is. It is like looking through whatever is in front of you rather than at it.
Hydromancy is the art of scrying with water. There are many ways to do this, the easiest being to gaze at a natural body water or a well and wait for messages to come to you. In Jake Richards' book Backwoods Witchcraft he discusses many methods of gazing including using a white porcelain bowl, but you can use whatever you have as long as it is a solid color. I like to use a cast iron pan with about a half inch of water in it. It provides a nice, shiny surface to gaze into similar to the modern Obsidian scrying mirror. Ideally, you would want to use water from a natural source, not tap water unless you have a well, but use what you have. Along the same lines would be scrying using a mirror. Mirrors mimic the reflective surface of water and make perfect scrying mediums.
Pyromancy is the art of scrying with fire. Scrying with fire is as simple as watching a candle flame or bonfire to receive the answers you seek. It is important to pay attention to how high the flame is, how long it burns, and how often it moves or dances. Other ways of scrying with fire include throwing objects into the fire such as hazelnuts, sheaves of wheat, corn, etc. Along the same lines would be anthracomancy (scrying with hot coals) and turifumy (scrying with smoke.)
The type of scrying that most people are familiar with is crystal ball gazing. This technique involves placing a spherical crystal specimen on a stand or piece of solid colored fabric and gazing through it to receive messages. I prefer to use crystals that are not perfectly clear for this. I feel like it provides more depth to a reading when a crystal has inclusions (another mineral occurs within the stone) or occlusions (an impurity trapped within the stone.)
I mentioned above that you need to soft focus in order to scry properly. To do that, you'll need to be in a meditative state where your mind is as clear as possible. Close your eyes and think of your question or problem. Meditate on it for a little while before opening your eyes and gazing into your scrying medium of choice. As you look into the medium, allow your gaze to soften so that you're not focusing on finding an answer. You want the answer to come to you naturally, so sit for as long as you need to. Make sure that you're in a comfortable position before you get started. Scrying isn't necessarily a short process!
Everyone sees something different while scrying. Sometimes it's letters, numbers, or other symbols that we're familiar with, but you might also see images such as faces or places, events, etc. Keep an open mind and a journal to jot down what you see so you don't forget it. Happy scrying!