I wouldn't recommend a dremel. It's a painstaking process of using an x-acto knife and going over the cut line many, many times over the course of a couple weeks. You want to minimize the amount of material removed so it is a seamless join between the two surfaces.
I know that some people cut up the blade from a jeweler's saw, hobby coping saw, or even a thin hacksaw blade to make a miniature saw. It needs a thin profile, regardless of how you make the saw.
The band across the top becomes a very fragile piece of the face. It tends to break every once in a while. If you have trouble getting your head into the helmet, try cutting out the ring at the bottom instead of this mod. I considered this mod for my clone helmet/costume, but I'm building one with binocs. I don't think heavy binocs + a face-off mod would go together very well at all...