Hey Chief -
Using a rotary tool like a dremel is recommended for the dirty cut. This is the cut you will make that stays about 1/16th of an inch or so away from the detail lines. A standard fiberglass reinforced cutting disc is a must. Don't use the cheezy discs that came with your tool, they will break nearly on contact. When you get to the tight corners, stop about an 1/8th of an ench before you reach the end of each side of the squint. Once you've cut the top and the bottom of the squint, leaving 1/8th or so of mmeat, you can just snap out the piece you have diligently cut out. After you've got most of the center knocked out, you can safely use a good flat file to get the rest, especially in the tight corcers. And in case you're wondering about the keyslots, a small flat file will be you're friend 
Just drill a number of 1/16th holes in each keyslot, esentially a "perforated line" in the center of each keyslot. Then, use your trusty little file to true the shapes.
That of course, was the "Sound Advice".
If you're good with the rotary tool, and have a steady hand, you can trim the visor out clean by using the detail lines as a guide, if done right, you won't alter the shape, and you will have very little clean up remaining.
Hope this helps.
On a side note, I don't usually recommend it, but I also use a "Hot Knife" It's a essentailly a 30 watt soldering iron with an exacto blade attachment ... VERY effective, especially on the keyslots. But VERY dangerous. Not only is it sharp, but it's soldering iron hot ... so it will practically cut through you .. almost like a lightsaber 
I suppose the bright side is .... you won't bleed to death ! It's cauterized :facepalm (unless it's an ANH original blade, in which case, you'll bleed like a stuck pig
FP
