This part is something you should be practicing along with meditation and sensing energy. I probably should have written this before doing the sensing energy post, but there's nothing that I can really do about this now.
Visualization is seeing something in your mind without it necessarily being real. This can be something like a memory or a dream, or it can just be an object in empty space.
To start off, get into a comfortable position and spend a few minutes winding down. When you're ready to start, close your eyes and picture whatever it is that you want. It is best to pick an image or a picture rather than a scenario to start off with.
It isn't uncommon for the picture to fade quickly in your mind, just keep bringing it back. Do this with the same image for 5 minutes or so, or until you start to feel tired or worn out.
It's good to practice this skill often, as it can come in handy for practicing magic. Once you are comfortable with a single image, turn it around in your mind, manipulate it in some way or another. (You don't need to be practicing with the same image every time unless you want to.) From here you can move onto places.
When it comes to visualizing places, it's something I usually do during meditation, anyways. The places I have came naturally to me. Try moving about it in a way that you would usually do. Avoid animals and people at first, as they will be difficult to manipulate along with everything else.
When you get to practicing magic, this will come in handy during the whole "visualize outcome while putting your energy into a candle" or another similar part of the ritual.
(A small side note: If you're a strong and vivid daydreamer, this should come naturally to you. Also, many good artists are very good at this. I, however, have issues keeping an image in my head.)