Decide which exam you want to take. If you are thinking about taking a DSST exam and are currently enrolled in college, check with your advisor to ensure that you will receive credit for the exam and it will be applicable to your degree program.
Go to the DSST website and download an exam fact sheet. These are available for free and outline which areas are covered on the exam and the percentage of the exam each area comprises. They also have a list of recommended study materials and about ten sample questions.
After you have decided on an exam and downloaded the fact sheet, you will want to locate study materials. Local libraries can be a good source for textbooks and often if they don't have a particular book they are able to get it for you from another library. There are also many websites, such as amazon.com and half.com, where people list their used textbooks for sale. Since textbooks are often needlessly updated, even the previous edition of a textbook will be enough to prepare you for the exam and older editions can often be purchased quite cheaply online.
Once you have your study materials, start studying! Try to devote some time each day until you feel comfortable with all of the subject areas that will be on the exam. Don't panic if you feel like you can't remember all of the information. These tests aren't designed for you to know the answer to every single question, they are an overall assessment of the knowledge that the average person has after completing a similar class. There will probably be questions that you can't answer, but don't be afraid to guess! There is no penalty for guessing and if you have prepared well, you can usually narrow the answers down to the best two, giving you a 50/50 shot at getting the question right.
Once you're confident that you are ready to take your test, you will need to locate a testing center that administers the exams. Many college testing centers offer the exams to anyone who wants to take them, whether or not they are a student. The DSST website has a list of testing centers, but be sure to call in advance to find out when testing is available and if you need to make an appointment. Most testing centers charge a $15-25 administration fee for the exam in addition to the $80 test fee.
If you didn't select a college to receive your results on the day you took the exam, don't panic! You can request a transcript of the exams you have completed at anytime from the DSST website.