They can work with circular polarizers as well. The linear polarizers are cheaper. Treat it like an ND2. Overall, it takes out half of the light -you just do not know specifically where-. Use a second polarizer for yourself, and mark it to align with the one on the camera. Your eye will not care if it is linear or circular, so get a cheap one. Rotate it until you like the effect, note the mark, and then line up the one on the camera using its mark.
How do you know if the marks are the same orientation if the filters are unmarked: Hold the two against each other. Rotate one of them until all the light is blocked. Make the marks at 90 degrees from each other, ie one at 12 o'clock the other at 3 o'clock. Now rotate the marks to line-up and the view through the polarizers should be at the brightest point. Do that, you can do the old "Otto Preminger" and look through the filter to visualize the scene.
