There are so many things to visualize. I would choose one skill to concentrate on at first and then add more when it's working for you.
Most people visualize to relax, learn a new skill, or to get rid of nervousness before a competition.
For example, if you wanted to concentrate on equalizing well to improve your equalizing efficiency - read this first if you haven't already:
www.liquivision.ca/frenzel.doc (there are versions in other languages at
www.liquivision.ca/liquivisiondownloads.html) - then you could concentrate on seeing yourself equalizing perfectly every time, your chin to your chest, body vertical, diving slowly to yourself enough time, and performing any skills you need to equalize effectively when it gets difficult.
Another example, which I've used for both constant weight and static apnea, before and during the breath hold, is to start from my toes and work up through every muscle, visualizing them relaxing and getting soft like jelly and being surrounded by a warm green glow. It doesn't really matter what you do, as long as it's simple.
Basically, decide how you want to feel when diving or identify which skill is most important to you and then practice it mentally. You can tell yourself how to feel if you concentrate and repeat it enough times. And chances are, you WILL feel that way during the actual dive.
You can visualize out loud, telling yourself how you will feel at a certain moment or what you will do. That can be effective. You can make a recording of how you want to make dive and listen to it with ear phones. Elite athletes often do this.
For example, I spent a whole winter imagining how to do a move in windsurfing and performed it on my second day back on the water in Spring without having ever tried it before! I was so surprised.
I use visualization for all my competition dives and even recreational dives, especially to remind myself to relax and accept the breath hold.
Good luck!