Right. FM2 has no TTL, so set the SB28 to Non-TTL Auto [A], set a desired aperture on the flash, set the same aperture on the camera, then shoot. Make sure the ISO on the flash and camera matches as well.
In older flash there is usually a dial at the back of the flash for you to check the setting. You just dial it according to your ISO and subject distance to get the aperture value. Modern flashes with auto mode will automatically calculate the power output according to the ISO and aperture value you set, with the distance it measures from its sensor.
In this case, since its not TTL... the flash needs to know how much light to output. The flash need to know the sensitivity of the film and the amount of light that is entering the camera. Then it fires a preflash (I guess) and judge the reflection of the subject and then emitts a flash that should correctly expose the pic taking the sensitivity of the film and the aperture into consideration.
i.e If the aperture is larger, the flash out put should be lower compared to a case where the aperture is larger. And a High ISO film will require less light, so the flash has to emit a lower output.