I believe that you can schedule it at any parametric center the same way you would schedule your normal exam. However, I do recommend scheduling it at the prometric center where you will be taking your actual exam at because the whole point is to simulate your test day experience and familiarize yourself with that particular prometric center prior to taking your exam. Many students find it very helpful in calming their nerves and minimizing any external variables that could cause added stress on test day (I.e. time it takes to get there with traffic, where to park, where the test center entrance is, what the center is like etc).
Annette G, MD, MPH - USMLE Tutor and Residency Advisor at Med School Tutors