Patricia Kain of the Writing Center at Harvard University points out that close reading begins with making close observations about the text. She advocates reading with pencil in hand to make notes about the details of what you are reading. You may wish to take notes about all aspects of the text, or you may decide to focus on some particular aspect of it. For example, you may especially look for historical references or instances where the author refers to mothers and fathers. Your purpose in reading will determine your focus.
Effective reading calls for previewing the text to attempt to get an overview of the author's major ideas. If possible, it's also a good idea to get the context for a piece of writing. If an essay was written in the early 1930s during the Great Depression, that contextual fact will help you understand the writer's attitudes toward Wall Street.
It's also important to be sure you understand the text as you read through it. College Board experts advise re-reading complex sentences or whole sections that are not immediately clear, as well as keeping a dictionary handy for looking up unfamiliar words.
The AP English test requires that you write on demand so it's important to practice that skill. While it's normally good practice to do classroom assignments ahead of time and let them sit overnight before re-reading and revising, you obviously can't do that on the test. Therefore developing good essay habits is essential. First determine the main point you want to make in your essay. Then carefully note aspects of the reading that can help you support that point. Be sure your essay is clear and logical. You can get practice by writing brief review-responses to editorials in the newspaper or to articles.
The AP English test calls for more than simply summarizing or responding to a single source. Some questions require that you use multiple sources, which the test provides, when writing your answer. It's important to practice this task of synthesizing ideas from multiple sources. In some school districts, this specific task is a regular part of the coursework starting as early as middle school. If your school district doesn't make heavy use of Data Based Questions, you will be at a disadvantage if you don't get in some extra practice on your own.