There are 4 prisoners you need to rescue. The first one is being held in the same cell where you found Paz during the Ground Zeroes main mission (in the basement of the admin building). The other 3 prisoners are all in the same spot. They are in the cells where you found Chico during the Ground Zeroes mission. I would advise to do the first prisoner, rescue him, and then go after the other 3. Not to try to do all 4 at once as sometimes enemies will go to the East beach after discovering the 1st is missing. All prisoners have to be alive when you extract them! Here is a video to aid you:
All credit goes to PowerPyx for his/her videos.