Its not actually a brooks... its an old ideal i got for free from our old seat box when I worked at the shop. To answer your question I just cut it with a set of sheet metal sheers... mostly cause they were the biggest scissors around at the time, and then I just cleaned up the cuts with a big rasp file... it was pretty spur of the moment, but like I said, if I had jacked it up I wasn't out any money. It was pretty easy. I wouldn't hesitate to do a brooks, but I would be a little more careful about it.
I think I would make an initial groove with an exacto knife and then just keep "scoring" it until I was through the leather... then I would clean up the edges with some 200ish grit sandpaper.
Hope this helps.
- Woody