I have done this so maybe I can help. If you don't have lots of dust on your sensor then first add dust to it. Make sure the background is light so that dust spots will show up well. Shoot at a small aperture, such as f16 of f22 and don't underexpose. Then in your editing software you can increase contrast, play with levels/curves, etc. to make sure the spots show up better. If you shoot jpeg and don't PP then set the contrast and sharpness to the highest level in camera.