Critical Analysis examines a given piece of writing (book, articles, paper homework, etc.), evaluates the arguments made by the author, and judges the conclusion in light of these arguments and examples.
To write a critical analysis, one must be able to identify the main idea/s, arguments made in favor or against it, theme used and critical points made. The writing mainly consists of evaluating the key areas that demand attention, organization & structure of work, ability of author to provide relevant examples/arguments, overall value of the work (esp. in light of available work in the area) and concluding with your judgement on the work.
It certainly helps to read the work and mark key areas and then revisit them for more clarity when you are actually writing. You must also be willing to research unfamiliar terms and should include other relevant work in the area. Some suggestions for improvements should also be included to fill in the missing gaps, information you feel would make a good addition to the work and will make it more comprehensive.