Easy, use manual flash mode along with manual exposure mode. Use a flash meter to determine proper exposure. Once you have that change flash power to adjust exposure to what you want OR, leave power setting as is and change aperture to adjust exposure. The nice thing about manual flash and flash metering is it ignores the reflectivity of the scene.

Example, bride in a white dress filling the frame. If you use a reflective meter, like the one in your camera, it will turn the white gown 18% grey. To make the gown white, open the aperture 1 to 1 1/2 stops to get white. Of course, using a incident flash meter ignores the reflectivity of the scene and always gives a proper exposure.

Tom
From Beautiful Smith Mountain Lake in Virginia