>>1603888A decent cable cutter, no diagonal pliers or walmart crap. Nothing fancy, just something with a little notch like this to trap the cable while you're cutting it. Shouldn't be more than like $10-$15 at home depot or whatever.
Also good to have a metal file to file down the ends of the cable outers so they're square and smooth. When you cut the brake cable outers you tend to have little spurs of metal that are hard to cut off neatly.