Exam sign-ups are handled through local exam administrators. Look on the Exam Calendar on the BJCP website to find a nearby exam. Email or phone the listed contact person and ask if you can reserve a seat for the exam. You should have completed the online entrance exam (provide the exam administrator with the certificate number you received when you passed) or be an existing BJCP judge (let them know you are a retake and provide your BJCP ID number).
Note that many exams fill up months in advance, so you may be placed on a waiting list or you may need to contact several exam administrators to secure an open exam seat. Often, people have to travel outside of their immediate area to secure an open exam seat. The BJCP also hosts exams at the annual AHA National Homebrew Convention.