Masking will not save swathes of deep, black shadow on Ilfochrome. This is especially true with contrasty emulsions like Velvia (less so with Provia). Nor will it save blown highlights. Look after the exposure at the time of exposure, in-camera and don't rely on quick-fixes in the print stage. It must be said this takes experience — a lot of it, judgement and understanding of the light and its effect on the scene. You are bound to be very disappointed when initially getting a handle on balancing highlights and shadows, but it will come easily and naturally to you if you expose the film in the conditions it was designed for.