You can calibrate the cameras one by one using a checker board pattern, which is the easiest method in my opinion. The process is very simple: first you have to load calibration images (capture the checker board pattern with the camera you want to calibrate), then you have to manually extract image corners, run the main calibration engine and finally you get the results.
If you don't have access to the camera calibration toolbox, I recommend you to use Caltech's algorithm. Here's the link: Caltech Camera Calibration
Hope this information helps.
