As I always mention, I am no expert at this, but I have read over and over again and also picked up some tips along the way from others who scry as well as with my own experiences. However--
1. Make sure you are relaxed when you scry.
2. Start off slow--and if you begin to feel ANY eye strain or eye pain or feel a headache coming on - STOP for the day!
3. Don't feel like you MUST light a candle, read only in a very dark room, etc. etc. I've read the ball while sitting in a car with my mother-who was driving--in the middle of the day--so it truly doesn't matter to have dimmed lights, candles lit, etc. etc.
4. remember that some pictures may come through as symbols. So, consider using a dream dictionary to see what the pictures could mean.
5. You do not need to keep your crystal covered with a black cloth--however, do not leave it out where the sun will hit it--as it actually CAN start a fire!
I believe it was on this site that someone here did or nearly did have a fire as a result of their crystal ball, sitting on a table in front of a window--with the sun shining down on it.
6. For most people--when you are close to getting things to come to you in the ball, you'll first notice 'clouds' inside the crystal ball developing.
Now, since you are a medium--you may have an easier time (or not) with scrying. I say this because you are at least use to receiving information clairvoyantly, and clairaudiently (hearing). I'm also a psychic medium and have clairvoyant, clairaudient, clairalient, claircognizant & clairgustant abilities.
Each of these abilties can also come through in my tarot readings as well as with scrying, so pay attention to ALL your senses/abilities and do your best to trust your instincts. After all, just like with the cards, the crystal ball is just a tool. The ability is actually inside of YOU, not the object at hand.
Enjoy it. Let it be fun. Don't get discouraged or upset if you sit down to scry and nothing happens. Nothing is 100% and some days are better then others.
There is a book that I always enjoyed and recommend to others--but right now I cannot recall the title--(and JUST now, her name came to me and the title!--
)
It is called: "Crystal Ball Gazing: The Complete Guide to Choosing & Reading Your Crystal Ball" by Uma Silbey. You can find it very inexpensively online, I am sure. I have it somewhere--and have read through it a few times myself. Loved it. However, as for Choosing your ball--that I feel is totally up to the person. Sometimes cost comes into play-and sometimes not. But like with anything you do--whatever you find and buy can work for you IMO. But sometimes they do literally seem to call you to them!
GotH has the most incredible stories - especially about her and her "Frankie".
eta: oops, forgot this question too-
both actually. You may see actual pictures and scenes like in a movie inside the ball happening. And, of course, you can see it actually taking place within your mind too.