You can sign up for it right now! I would do it at least 60 days before you want to take it to make sure you get the day you want, but most test centers allow you to take it any day of the year either in the morning or the afternoon, so it's not as hard as the scheduling the MCAT. Once you pay to take the exam, they will mail you something and you can go online and schedule the test at any Thomson Prometric Testing Center.