I personally do this by setting the camera for the correct ambient exposure then I will us a flash with a built in light sensor such as a Vivitar 285 or 283. The flash will then be put on a setting to give one or two stops less light than needed.
There are two ways to do this. Either set the ISO on the dial one or two stops lower than the actual film being used or use a setting for an aperture one or two stops more open than actually set on the camera. The Vivitar flash has multiple coloured settings on the dial which make this an easy option. Not all flashes are this versatile.
Another advantage of the Vivitar is that you can unplug the sensor from the front and connect it with an extension cable so the flash can be away from the camera with the sensor at the camera.