There are some spells which are short, simple, and don't involve anything akin to a meditation other than just centering yourself enough that your mind isn't scattered all over the place when you're trying to work your magick. Other spells, particularly those which involve trying to draw you closer to a deity or make you more attuned to them, will have some portion which is essentially meditating on the invoked forces (even if the spell instructions don't expressly call it a meditation).
There's no single answer to how you meditate because there is no single way to perform meditation. Asking how you meditate is sort of like asking "how do you cook?" Some involve focusing on a word, phrase, or image to the exclusion of everything else. Some involve focusing on something internal, like your own breathing, to the exclusion of everything else. Zazen meditation is essentially focusing on nothing .
Focusing on your breathing is a very good beginer's meditation, and its common in many traditions. Some traditions will prescribe certain ways of breathing during the meditation, such as the fourfold breath (which you can read about here; http://tinyurl.com/kxrg29g).