It appears the current process is to start the application process and pay the fees, but there does not appear to be requirements beyond that to be eligible to take the exam itself. According to the diagram posted, the portfolio review is concurrent with the exam and there is a 6 year window to complete all the requirements.
GISCI > Applicants > Application Information
However, I would review the full process on the GISCI site to be sure, as I may have missed something (as an existing GISP I don't have access to the first-time applicant process login, which may contain more information).
Some relevant GISCI links:
GISCI 2017 Fall Exam Application Period Continues
Exam info for candidate
https://www.gisci.org/Portals/0/PDF
If you have any uncertainties after reviewing the posted info on the GISCI site, I would contact GISCI with any questions.
Chris Donohue, GISP