I just did this a month ago. I can add some tips:
Cutting the diagonal lines is pretty easy, but I found when cutting the lines parallel to the louvers could be tricky. My advice is to leave a few extra louvers and then trim the side even with a smaller pair of clippers (that are capable of cutting the fine brass wire that bind the louvers together). You can trim those sides up pretty neatly (and even)
I also found that if your piece is too wide (and you should cut them a little wide), instead of trimming the sides, you can use the small clippers to trim the top or bottom of the piece and simply shift the piece up or down until it fits (since the sides should still be at the correct angles with respect to the top or bottom)
The edges of the koolshade WILL bend. Not much you can do but bend them back. The plus side is most of that bending will be covered by the wing stars.
I still need to post pics of my TIE, but feel free to hit me up on PM.