To get started you will need a candle, a small mirror, and a dim/dark space in which to experiment. No need to dress special, but do feel free to get comfortable. You may want to banish or cleanse the space you're working in, or to do this within sacred space, whatever you decide will be fine.
Once you've gotten settled and are ready to start scrying, you'll need to spend some time just getting yourself relaxed. This is a common element of many magickal practices. Take your time.
When you feel that you are relaxed enough and ready to proceed, set the mirror in front of you. Flat on a tabletop, altar, old stump, or packing crate is fine. I'm not picky. Now set the candle so that it is situated at the top of the mirror and light it. If you haven't already, now's a good time to turn off the lights. You should be able to see the reflection of the candle flame clearly without discomfort. Adjust yourself so that you are comfortable and relax some more. Don't fall asleep, but do get into a position that will allow you to be still for a while.
Look into the candle flame's reflection in the mirror. Don't look at the flame, but into it. Allow your eyes to adjust a bit. Relax.
Once you've gotten used to looking into the flame, it's time to shift your vision a bit and to look through or past the reflected flame. Feel yourself flowing into the flame within the mirror. Let the flame fill your vision completely. Immerse yourself in it. Relax.
This may take a few tries. Don't sweat it. Take your time. Have patience. Stay relaxed.
Once you have been able to feel yourself flowing into the reflected flame, your vision shifting into a place that is neither in the flame nor in the mirror but rather someplace else, you're ready to start scrying.
You are now in a place, or rather experiencing a state of perception in which you can see anything you want. You just have to direct your vision and you'll see what there is to see. At first this can be both unsettling and a little unnerving. You're not used to this way of seeing, and it can tire your eyes and you may well get a headache the first couple of times you try this out. Like anything, if it hurts, stop doing it. Don't strain. Take your time.
This is just one way to jump-start this faculty. There are plenty of others you can try. For example, you might replace the mirror with a bowl of water or oil. You can stare directly into a fire such as in a fireplace, camp fire or whatever. You can get results looking at the snow falling outside your window or the white snow of a TV channel with no programming. You might want to experiment with crystals, amber, or anything with an elaborate, stimulating pattern. You can also scry using Tattwas, Tarot cards, illustrations, diagrams, I-Ching hexagrams, or sigils. It's a matter of finding what works for you.
After scrying it's a good idea to ground yourself with a little exercise, some food, or a good night's sleep. You might find it helpful to record your results in a journal. Once you start keeping track of things, you may find that you are seeing more than you realized at first. This is a subtle process, at times, other times it can be overwhelming. It varies according to circumstance.
Once you begin to explore this aspect of perception, you need to keep in mind that just because you saw it in a peculiar state, it isn't necessarily true, nor is it guaranteed to happen. Scrying is a combination of visualization, imagination, concentration and intuition. It is by no means infallible. Just as you eye can play tricks on you, so can scrying.
Lastly, once you've tried to scry and gotten some familiarity with it, you might consider how you would go about trying to see' things similar to what you perceive when scrying, but without using your eyes or any sort of physical visual stimulation. This is where scrying can lead to developing clairvoyance, prophecy, astral projection, and similar abilities, at least according to some authorities.